It's not unfounded. The GCC compiler isn't splitting up because they fear someone would build a proprietary compiler using either backend or frontend API if they did it.
I can see that the same reasoning here would be valid; someone could write a proprietary GPG frontend.
You have one example of a project not providing an API, however there are tons of other GNU projects which do.
Furthermore the GCC decision is well documented in mailing list posts, have you ever seen anyone involved in GPG development claim that they won't allow a library/frontend split for fear of someone writing a proprietary frontend?
I can see that the same reasoning here would be valid; someone could write a proprietary GPG frontend.