....what's the fundamental difference between what these researchers are saying and what James Watson said a couple weeks ago?...

Agreed. What is the difference other than complementing a "race" rather than denigrating it.

On the other hand, if intelligence is a basis for improved survival and propagation, then why are Jews so consistently a small fraction of the world's population?

Estimates of intelligence are more likely a measure of how well an individual understands and incorporates the elements of human society (language, urbanity)