Dropdown list created from a list in one cell

Not possible via Data Validation. You can either use List of items mod where you enter your comma separated values directly, or you can use List from a range mod where each dropdown item needs to be in a separate cell.

A workaround would be to have your one cell with CSV items as you wish to, and then secretly deploy that one cell into a range and feed your Data Validation from that range referencing your one cell. Something like this:

A1: dropdown
C1: comma separated values in one cell
E1: =TRANSPOSE(SPLIT(C1, ","))

This isn't a built-in Google Sheets web app feature but by using Google Apps Script it's possible. To learn the basics, please checkout https://developers.google.com/apps-script/guides/sheets