Manu's ranking is just fine. As many here have noted, he's really tough to rank because of the unique cir stances around his arrival and career (late start, willingly coming off the bench for the sake of strategy).
In making these lists, I still think you have to go with guys who have the individual body of work like AI over Manu, because dealing in "well he could have" arguments is pointless. However, there's no way in those 2000s Spurs teams would have been as successful if you swap AI with Manu.
Manu was put in a great situation and also made personal sacrifices for team success, the same way Tony and Timmy did. Those 3 were perfect for each other, and I wouldn't have had it any other way. Let's not forget that those Spurs teams were a few bounces away from even more les. I'm super proud to say I was about to watch all 3 together in their primes.