The Post Comments Form block is currently implemented to be rendered by comment_form() on the frontend (which can be fairly complex, containing several different labels, messages, and the form itself in different versions for logged in and out users.

In the editor, it is just a placeholder.

We need to match what is editable in the editor with the output produced by the frontend, by replicating the logic of the comment_form() function (see also #22724 for a breakdown of other possible functions affecting the comment form).
All its parameters and the comment form filters (comment_form_default_fields, comment_form_field_{$name}) should be taken into consideration.

Props to @ockham for #24101, which inspired this issue.