I think the point he was trying to make is that there's a great deal of genuine Mahler in M10 that's worthy of our attention, despite the sometimes bare nature of the sketches, so it shouldn't just be written off as an unfinished, unperformable work, which was the position taken by some well-known conductors. I'm not a Mozart expert, but I think there's a lot of doubt about how much of the Requiem is genuine Mozart, although I admit that as a whole it hangs together well in the Sussmayr completion. Of course Mahler would have made substantial revisions, but thanks to Cooke et. al. we have a wonderful symphony, even in its incomplete state. Who can deny that?
Anyway, whatever your feelings, I think you'll enjoy hearing the exchange of views in the BBC programme (it's actually more of a discussion between Stephen Johnson and the host Andrew McGregor).