Thank you, EP, for being 'someone'.  I should remember that you are the one who can current do these tests.  I presume the reproducible 'this' is the non-response to cmd-A, cmd-Z, and so on.  So closing as 3rd party, tcl/tk, issue.

I comfirmed that Jacob's expectation is reasonable in that the keys do work in the macOS save-as dialog opened from Safari (and I presume in other Apple-supplied apps).

The tk doc https://www.tcl.tk/man/tcl8.6/TkCmd/getOpenFile.htm says merely "pop up a dialog box for the user to select a file to open or save." Some details are system-specific.  The only promise about user interaction is that one can enter a new name when saving and either confirm or cancel.

The tkinter doc https://docs.python.org/3.10/library/dialog.html#module-tkinter.filedialog adds "native look-and-feel", but this is an interpretation of the intention of the tk function and is completely dependent on them.  Any failure of the 'feel' part should be reported to tk.  (The IDLE doc merely says "with a Save As dialog".)

The IDLE Edit menu only applies to its editable text windows.  Their menus are grayed out for modal dialogs.  Tkinter dialog entry boxes come with selection and clipboard hot keys, but not undo keys.