Well, I wouldn't say it's in any way a "rookie" thing. I personally believe the idea of Dabura likely being stronger due to some statements on SPC's power compared to Gohan's, matters concerning Dabura's comparison to Cell and implications on Dabura potentially not going all out against Gohan, but I wouldn't say it's anywhere close to such a clear-cut matter for believing Gohan to be a SSJ or how Dabura compared to Boo Arc Gohan to say either viewpoint is a "rookie" one.
In terms of arguing against the point in general, I'd just copy-paste what I said in
this thread and add in that Vegeta doesn't comment on Gohan being much stronger in general as a kid, only commenting that he was much stronger when he killed Cell (which was great enough for him to be closer to Boo Arc Goku/Majin Vegeta than to Pre-Majin Vegeta).
As for the 450 million matter, that's contradicted by it being very apparent Goku got far, far stronger since the Freeza Arc where his SSJ self was already at 150 million, making SS2 Gohan only having 3x as much Ki logically impossible.