Note that a16z invests in startups at a much later stage than YC does. For YC, the quality of the team may be the most important because they're at a stage early enough to significantly change the market they target. By the time a startup is looking for series A (or later) funding pivots are relatively more expensive.
Yeah, I suspect that YC is willing to invest based on team because they figure the idea can easily change anyway. I've heard that one of the points of the YC interview is to weed out inflexible founders that are not willing to radically change their approach. And Andreesen has recommended that "Until you achieve product/market fit, keep the team as small as possible.".
