Yeah you gotta think about what the "this" is in Decker's declaration. Particularly since three days ago, he didn't even know about V'ger.
The novelization indicates that they do understand that V'ger is preparing to "unite physically with the Creator" and suddenly Decker is intending to be the person to join with the entity. Kirk wants to be the person to decide who gets to do it. Spock tells him to let Decker proceed because Decker "knows exactly what he's doing."
It's only after the "transcendence" (as Spock calls it in the novelization) has started that the Ilia Probe steps into the event, so Decker is certainly not doing this
just to be with Ilia again.
What "ST Phase II" was
intending to do with Decker, over the mooted series: to develop the backstory of his mother's membership in the New Human Movement (also mentioned in the novelization), a quest for humans to develop a hive mind.