"Well, the memo from Bob Justman of August 9, 1967 only indicated the names he liked from D.C. Fontana's proposed list. I don't think it was meant to be the exhaustive list. For example, it doesn't have the Defiant which was still a year from being created, but his list also didn't include the Exeter--which was among D.C. Fontana's suggestions. In the end, it looks like the list of Starships was settled upon by Roddenberry regardless of what Bob Justman's preferences might have been."
That is probably the case and I have little doubt that it had probably been the script for "Space Seed" that was the inspiration to add a starship Constitution
to the final list, but nothing more and nothing less.
"Of course, the "Constitution class" script comment first appeared in the "Space Seed" script by Carey Wilbur and Gene L. Coon. It's there not only in the Second Revised Final Draft script dated December 13, 1966, but also in the Final Draft script dated a week before that, if not in earlier drafts."
But regardless what Carey Wilbur and Gene L. Coon may have tried to accomplish with that odd "Constitution Class" script line (the producers hadn't even settled for the names of the other 12 starships like the Enterprise) it does not reflect one bit in The Making of Star Trek
(although that should have been of interest to the author Stephen E. Whitfield considering his job was to generate PR on behalf of AMT and future sales of the Enterprise
But in this book from 1968 we have the Bob Justman memo refering to the "Enterprise Starship Class" and
an even clearer description: "The Enterprise-
class starships..." ("Mission and Men", chapter 3, page 203).
Whether Walter Matt Jefferies shouted "veto" when he heard about this idea of a "Constitution Class" is unknown, but it's obvious that by 1968 the creators / producers hadn't settled for a "Constitution Class", otherwise the book wouldn't refer (twice!) to an "Enterprise Class".
I think the one thing we can all agree upon is that the Enterprise
is a member of the Starship Class - and even if it were simply for the fact that this is another quote straight from this book.