There's still the issue of "Sarek" (TNG episode, not the novel) which states that Perrin is Sarek's second wife. I've always assumed that's an official decanonization of TFF since the alternative is that Sybok's a bastard.
All we have is Picard's log entry referring to Sarek's "first wife" as being from Earth. All that proves is that Picard
believed at the time of that log entry that Amanda was Sarek's first wife. Picard's belief could easily have been in error. Given that Sarek had disowned Sybok and that hardly anyone, not even Spock's closest friends, knew that Sybok even existed, it's likely that the general public is unaware that Sarek had a mate before Amanda.
For that matter, we know nothing about Sybok's mother except that she was "a Vulcan princess." It was never stated that she and Sarek were actually married, just that they bore a child together. Even among Vulcans, it is no doubt possible to bear children out of wedlock. Or perhaps their marriage was dissolved in a manner analogous to annulment, in which case it would officially have never existed.