I have edited my previous post to remove that clause, since after a bit of searching, I have determined that you are correct: despite the idea's prevalence in fan discussion, especially in the 1970s, there is no canonical proof for the idea that Spock was the first Vulcan to sign up for Starfleet.Where is it established that Spock was the first Vulcan in Starfleet? His notoriety could come from simply having a high ranking Ambassador marry an Earth woman and having a son. Next add in that said son (Spock) elects not to follow in the steps of his well known father to join the human centric Starfleet.
This is pure media and tabloid fodder or the 23rd century equivalent on Vulcan.
The absence of that clause doesn't seem to change the primary point about Pon Farr that I was making, anyway.