Can’t save edits to posts or pages when REST API is disabled.

I asked on a FB group and somebody replied this:

“FluentAuth plugin also has option to Disable REST Endpoint for wp users query for public and you are able to save when it is disabled.

it very well could be, but I’m not the developer of either plugin, so.. 🤷</img>
Hypothetically speaking, FluentAuth by name alone suggests it is focused on Authentication matters. If that’s the case, that developer could have made that disabling of REST conditional on authentication status.
If (authenticated user) { allow rest } else { rest die };
[not real code clearly, just for explanation purposes]
The plugin OP has linked to doesn’t appear to make any conditional consideration. It’s just on or off.
Either way, those files aren’t going to be encrypted or obfuscated, you should be able to see if it’s conditional pretty quickly.”