Well, like I said: I don't know (or much care, really) what the in-universe import is of the "BASIC SPECIFICATIONS, CONSTITUTION CLASS STAR SHIP" comment in the "Space Seed" script. I only point out that Greg Jein didn't simply fabricate the Constitution-class notion out of whole cloth by himself. I think there's value to knowing that it was actually scripted
. way back in 1966.
If the creator of Star Trek approved a script that indicated that the Enterprise
was a Constitution-class star ship, folks might well consider him to be the ultimate authority on the subject--rather than the simple Art Director. I can understand Greg Jein and (later) Franz Joseph Schnaubelt taking The Great Bird of the Galaxy at his word.