Sipa is the only person still working on Bitcoin on that list, and is therefore the longest serving programmer in Bitcoin development. So Sipa has probably contributed to Bitcoin development more than anyone else.
I'm pretty sure that sipa was only added to that list so that he could help migrate/maintain the bitcoin-dev mailing list.
Hmmm, invalid supposition is invalid. Amended my post.
Can a mod give buwaytress his merit back? Turns out I shouldn't have been given it.