I think the conclusion Joseph and Jein came to - independently - was that "MK IX/01 CONSTITUTION CLASS" referred to the class to which Enterprise belonged. Because only one slot precedes "1701" - 1700 - the deduction is that number belongs to Constitution. No need for one to copy conclusions from the other. And since they were both basing their conclusions on something that had made it to the screen, it took precedence over what had been printed in TMoST. Thus Enterprise-class becomes Constitution-class for both Joseph and Jein.
Joseph didn't need the Star Trek Sketchbook to know what Jefferies meant by "1701" being the first built of the 17th design. He met the man. Obviously some accomodation of the "Space Seed" phaser plan had to be made. But the general idea that the first numbers are linked to the design persist, since FJ goes to pains to do just that. Unless of course he came up with the same idea independently.
I wonder which people "that fully knew" you have in mind.
Mike Okuda. He knew Matt Jefferies.
Like I said before- I think this boils down to loyalty. Loyalty to Roddenberry and to Jefferies. There is evidence that by the late 70s neither liked what Franz Joseph did. Maybe it was resentment of the money he had made. Maybe it was resentment of the credit he was getting. Maybe it was the way he was redefining what they'd done. Who knows? But it makes sense to me that Mike Okuda was being loyal to GR and MJ by not giving credence to FJ... despite the fact FJ's system was based on Jefferies' own idea! What I'd like to know is why go with Jein? Why not stick with the Jefferies scheme and just abandon the specific assignment of names to numbers that FJ had made? THAT is the mystery to me.