Essentially the 'isList' Boolean check here results in the wp.media frame created always being in a 'single select' state, whether called by a file_list or file field type.

Re-writing as multiple: isList ? 'add' : false results in the file_list media frame allowing multi-select as I would expect.

I get somewhat confused looking at the WordPress core functions around this, in media-views.js there are various references to the multiple option being both Boolean and supporting 'add' or 'toggle' string values. Bit beyond me TBH, a little more background on this SE question.

The upload_file language string should also be updated to reflect being used on both single and multiple uploads, currently Use this file for both, simplest would be Insert File(s), rather than a whole new language string being added and checked for on the button: option. "Insert" also follows the common syntax of WordPress rather than "Use" or "Add".