Considering that a highly detailed character usually doesn't surpass 200k polygons, 6 million polygons doesn't make sense at all.
btw
720*1,280=921,600
It make absolutely no sense to me to have a 6:1 ratio between polygons and pixels.
We've heard of people talking of 3M+ polys per frame already, that'd be a 3+:1 ratio roughly going by those calculations.
For comparisons sake, jak and daxter 1's island(whole world) is said to have 50+M polygons for the whole thing.(around 10 hours to beat that game, but IIRC, there are several things to do in the various areas, so you re-thread same territory for quite a bit of it.)
I haven't played uncharted, so I don't know how long or short the jetski segment is, which would help us here.
I do know the interview is a bit ambiguous, he's saying the level is one of the biggest in uncharted, then mid sentence gives the 6M poly figure and starts talking about realtime reflections and bump-mapping, shadowing, refractions, etc.
which would hint at it being things running in realtime, and thus per frame.
Again, the beginning of the sentence would suggest it's is whole level, but the latter part hints otherwise. But if it is one of the biggest levels in uncharted, it might be bigger than jak's island and thus should have more polygons than jak's island.
I'd have to know how big it is to say for sure, maybe it's the biggest by a little or maybe the levels are small in uncharted, in which case the 6 million would be for the whole level.
edit:
But the figure is a bit too high, as you've said, so it is likely it is for the whole level.