That's the consensus from draft scouts. I could post a half dozen more scouting reports on Ta'amu that say basically the same thing. You're the outlier. Not saying you're wrong; you've watched him more than I have. But he's generally regarded as a nose tackle prospect, not just due to his size, but also because of how he plays.
Oh believe me, I know you could. I still disagree. I think what primarily lead to him being projected as a NT is his size and bulk and not because of the way he plays.
I remember early in the season last year, I started watching as many college games as I could. I wanted to scout for a few key positions; QB, NT, CB primarily. I watched as much as I could of Washington, Memphis and Alabama (among many other teams) primarily to watch Ta'amu, Poe and Chapman. I wanted to like Ta'amu from the get go but the more I watched of him, the more "meh" I felt about him. After the first game I watched of Washington and then Memphis, I was highly impressed with Poe (who at the time was being considered a 3rd-4th round prospect) and was not really impressed with Ta'amu (who at the time was being called a sure fire 1st rounder).
Like I said before, if he can be coached up, a lot, then he could be a potent NT. He does have the size and frame for it and I'm sure he's much stronger than he showed when he played. I'm also sure that had a lot to do with using improper technique and not knowing how to use leverage to his advantage. If he can be coached on these things then he might be able to be very good. However on the flip side, I also still believe that if he were to get down to the 300-310 range then he could also play the 5-tech or 3-tech positions.
Read everything in all those draft reports on him. Many of them also speak of him having a great burst off the snap but low top end speed and poor endurance, which is understandable for a guy his size. I just read a few different draft profiles on him that spoke of his burst allowing him to penetrate a gap, but his lack of speed prevents him from really being able to use that burst to his advantage. So if he were to lose some of that weight, then that should help to improve his top end speed as well as his endurance. If that were to be the case then, as I said, I think he could be a very good 3-tech.