I agree that it fits the general understanding we have of oneness, that's not the problem I have with this at all.
The only issue is that if we consider that specific moment as a oneness state, then we must downgrade that ""ability"", since the other two times he used it, he didn't get tired or had his energies fading. Heck, even weak (for Luke's standards) Jedi got ultra instinct and super sayian power levels when they enter oneness and they never, not once, do they tire, the only other instance of something like that happening is when Ganner Rhysode enters a oneness state when his body was already decaying.
If we accept Luke's malestrom moment as him having merged with the Force then we must also agree that Abeloth (against whom Luke did NOT tire in the specific battle where he attained oneness) is weaker than random Vong slayers, which is maybe the most preposterous statement someone could make in SW Legends powerscaling.
Furthermore, Revan has a similar statement in his novel, I think it's when he fought against Darth Nyriss. One time he was described as "the heart of the Force", a statement I think all of us know about and also another one which I don't recall that well, but it was similar to the one made to Luke in that Vong battle. The point I want to make is that Revan never achieved oneness, and I think no one has any doubts about that, yet has similar statements. Even "being the heart of the Force" could well be equated to being "a malestrom of luminous force energy".
Again, don't take this as a personal attack, this is just my view.