This cannot be correct Prime Sarek disapproved of Starfleet. Unless he is not a part of the crew plus he would be on Vulcan with his wife and teenage son Spock.
- James Frain as Sarek: A Vulcan astrophysicist, and the father of Spock. Frain appears as a younger version of the character who was first portrayed by Mark Lenard in the original Star Trek series.[10][11]
According to Memory-Alpha
A scene cut from the final script of "Journey to Babel" established that Sarek was previously an astrophysicist before becoming a diplomat. (Star Trek Compendium, 4th ed., p. 89)
So they're canonizing an previously cut fact.
And who said he was serving on the ship? He could just be reoccurring.