Respectfully, no, on the post strike "requirement".
I have a post strike Marti verified non-GT S code.
While non-GT S codes are not the majority, they are reasonably common. I think most putting up the $ for the S engine probably wanted a bit more fun and would assume most would have gone for the GT option as well.
But the GT option and the 390 were never mandated as a pair.
As for Ford docs, as they were printed long in advance of the model year many aspects are just not correct. Like the mythical 427 in a Mustang in 68. Mentioned in many Ford docs, never built.
I have no idea why this myth refuses to die as the facts clearly show the S code was never required to be a GT....
Here is a pretty convincing Ford document. Note the last line in the far left column and the date on the far right bottom.