Why using @ inside newcommand gives "Undefined control sequence" error?

I’m trying to create a command that temporarily changes the size of a lstinline, so that I can properly use it in a footnote. The following commands seem to work fine:

makeatletter
lst@AddToHook{TextStyle}{letlst@basicstyle=footnotesize}
makeatother

However, if I try to add them on a newcommand and then use it:

newcommand{testcmm}{
    makeatletter
    lst@AddToHook{TextStyle}{letlst@basicstyle=footnotesize}
    makeatother
}
testcmm

I get the folowing error:

! Undefined control sequence.testcmm -> makeatletter lst@AddToHook{TextStyle}{let lst @basicstyle=n... testcmm

I know that this error usually happens when the command is not defined, for example, whan you forget to add the library that contains it. In this case however, it only happens inside the newcommand. Why?

---

MWE:

documentclass{article}
usepackage{listings}

newcommand{testcmm}{
    makeatletter
    lst@AddToHook{TextStyle}{letlst@basicstyle=footnotesize}
    makeatother
}

begin{document}
testcmm
end{document}

! Undefined control sequence.
testcmm ->makeatletter lst
                              @AddToHook{TextStyle}{let lst @basicstyle=f...
l.12 testcmm

lst@AddToHook

documentclass{article}
usepackage{listings}

makeatletter
newcommand{testcmm}{%
    lst@AddToHook{TextStyle}{letlst@basicstyle=footnotesize}%
}
makeatother

begin{document}

testcmm

end{document}