The simplest test case I can think of it to add the editor to the comments form, which I did via this code in a twentynineteen child theme with completely stock content (using the Hello World! post to test).
The mediaButtons setting does act to show/hide the button itself, but is not functional.
I have the problem on a 4.9.8 site (custom theme), both on the comments field and on other textareas I have tried. The simplified test (above) was 5.0, and the same symptom is exhibited with and without the Classic Editor plugin enabled.
Does anyone have any ideas to try, or would anyone have a configuration where instantiating the editor via wp.editor.initialize() results in a working 'Add Media' button?