This command (whose origin and rationale may be lost in the mists of time) does not do what you expect and only loosely corresponds to the emacs command (read-only-mode) typically bound to C-x C-q. Hence, it is not one of the documented commands. The intent was to allow some editing functionality in buffers (not necessarily editor buffers) that were marked as read-only as a temporary convenience; but even there, the implementation is a bit buggy.
Admittedly, it would be nice to have that functionality, but back when this was implemented, I didn't have the hutzpah to take it on (while it's possible that it could be relatively simple to implement, it could also turn out badly).