Can I avoid indentation of margin items when using parbox=false in a tcolorbox?

I propose another approach (much easier to understand) to put the theorem boxes on the margin.

Using the package xcoffin you can put any material anywhere on the page. The reference point is the current insertion point.

Because there are some theorems that come from inside a tcolorbox and others from the text area, it takes two macros to account for the different margins involved.

(1) insertTheoBox{<content>}{<vertical shift>}

(2) insertTheoPara{<content>}{<vertical shift>}

The vertical shift allows for a vertical displacement inside the margin area.

Two inner "boxes" are used in the macros: Framex is a zero depth box that allows for the Theox box to be joined at a proper position.

The result of the assembly is then typeset into the margin area.

UPDATED Minor issues corrected. No hardwired dimensions.

%%%% version 3

documentclass[twoside]{article}
usepackage[breakable, theorems, skins]{tcolorbox}
%tcbset{enhanced}
usepackage[showframe,letterpaper]{geometry}
usepackage{ifthen}
usepackage{lipsum}
geometry{inner=1in,textheight=9in,textwidth=320pt,marginparwidth=150pt,marginparsep=20pt,bottom=1in,footskip=29pt}

newtcbtheorem[number within = section]{myexample}{Example}%
{breakable,
    colback=white,
    colbacktitle=white,
    coltitle=black,
    parbox=false,
 %  boxsep=14mm,  % for testing
}{ex}

newtcbtheorem[number within=section]{myfig}{My Theorem}%
{colback=white, colbacktitle=white,coltitle=black,
    fonttitle=bfseries,
}{th}  


%%*************************************************************** v3
usepackage{xcoffins} %added
NewCoffinFramex
NewCoffinTheox

usepackage{changepage} %added
strictpagecheck

newlength{Textw} % save textwidth outside the boxes
setlength{Textw}{textwidth}
newlength{Hshift}
newlength{Mshift}

newcommand*{calculateMshift}{%
    checkoddpage
    ifoddpage
        setlength{Mshift}{marginparsep}
    else
        setlength{Mshift}{dimexpr-marginparsep-textwidth-marginparwidthrelax}
    fi}

    newcommand*{calculateHshift}{%
    checkoddpage
    ifoddpage
        setlength{Hshift}{dimexprTextw/2-tcbtextwidth/2relax}
    else
        setlength{Hshift}{dimexpr-Textw/2+tcbtextwidth/2relax}
    fi}
    
newcommand{insertTheoBox}[2]{% v2 updated         
    par %start a new line
    calculateMshift
    calculateHshift    
    SetHorizontalCoffinFramex{color{blue}rule{tcbtextwidth}{0pt}} %clear box Framex
    SetVerticalCoffinTheox{marginparwidth}{#1}% fill box Theox
    JoinCoffins*Framex[r,vc]Theox[l,vc](dimexprMshift+Hshiftrelax,#2)%join boxes
    noindentTypesetCoffinFramex[-2baselineskip] %typeset assembly
}

newcommand{insertTheoPara}[2]{% v2 updated
    par
    calculateMshift    
    SetHorizontalCoffinFramex{color{red}rule{textwidth}{0pt}}
    SetVerticalCoffinTheox{marginparwidth}{#1}
    JoinCoffins*Framex[r,vc]Theox[l,vc](Mshift,#2)  
    noindentTypesetCoffinFramex[-2baselineskip]
}
%%***************************************************************

begin{document}    

    begin{myexample}{title}{label}
        1. lipsum[1]           
        insertTheoBox{%
            begin{myfig}{title1}{label1}
                begin{itemize}
                    item An item
                    item Another item
                    item A somewhat longer item that is a bit longer than the margin width
                end{itemize}
            end{myfig}
        }{70pt}         
                
    2.  lipsum[2]          
    insertTheoBox{%
        begin{myfig}{title2}{label2}
            begin{itemize}
                item An item
                item Another item
                item A somewhat longer item that is a bit longer than the margin width
            end{itemize}
        end{myfig}
    }{50pt}
            
    3.  lipsum[5]      
    end{myexample}

    insertTheoPara{%       
    begin{myfig}{title3}{label3}
        begin{itemize}
            item An item
            item Another item
            item A somewhat longer item that is a bit longer than the margin width
        end{itemize}
    end{myfig}
        }{-40pt}    

    
    4.  lipsum[1-2]                    
    insertTheoPara{%
    begin{myfig}{title4}{label4}
        begin{itemize}
            item An item
            item Another item
            item A somewhat longer item that is a bit longer than the margin width
        end{itemize}
    end{myfig}
    }{-20pt}
    
    5.  lipsum[3]  
    
        insertTheoPara{%
        begin{myfig}{title5}{label7}
            begin{itemize}
                item An item
                item Another item
                item A somewhat longer item that is a bit longer than the margin width
            end{itemize}
        end{myfig}
    }{-250pt}
    
    begin{myexample}{Title5}{label5}
    6.  lipsum[2]  
    
        insertTheoBox{%
        begin{myfig}{title6}{label6}
            begin{itemize}
                item An item
                item Another item
                item A somewhat longer item that is a bit longer than the margin width
            end{itemize}
        end{myfig}
    }{60pt}     
            
    end{myexample}
end{document}

The margin boxes and their positions will automatically adjust to changes in the page geometry or the internal geometry of the boxes in the text area.

For the next figure

geometry{inner=1in,textheight=9in,
    textwidth=220pt, % changed <<<<<
    marginparwidth=220pt,  % changed <<<<<
    marginparsep=40pt,  % changed <<<<<
    bottom=1in,footskip=29pt}

    newtcbtheorem[number within = section]{myexample}{Example}%
{breakable,
    colback=white,
    colbacktitle=white,
    coltitle=black,
    parbox=false,
    boxsep=14mm,  % added <<<<<
}{ex}