I read that story as Fontana absolutely felt that Spock was an only child, as that was her original intent when she was laying out most of Spock's history during TOS, and she smiles and winks that Therin was sharp enough to catch her little in-joke rebellion. It'd be smart for her not to say more in the way of an explicit confirmation, as she wouldn't want that line changed in subsequent printings (Don't forget that 1988-89 was during the Richard Arnold era)."She smiled and winked."
I don't know, you could be right, but that doesn't sound like an explicit confirmation to me - sounds like a writer having fun with a fan (in a nice way) ... maybe an acknowledgement she felt the same way. The calendar seems wrong for it to be so, as TomSwift says above... but, I wasn't there, and Therin was, and I don't have any explicit proof of my hypothesis, either.
I'm not sure of the exact timing of the writing of Vulcan's Glory, but my copy is copyright 1989, so it was likely being finished up in 1988, when STV:TFF was in the script stage or shooting. I'm now wondering if Fontana could've been writing the novel in the first place because of the 1988 Writer's Guild Strike, which spanned from March 7 to August 7, 1988. The strike would've freed up a significant chunk of time for her, during which she couldn't write anything for TV or film. Just about the right amount of time to turn out a Trek novel, though...

But maybe Therin can shed more light on this when/if he chooses to chime in.