Though this is Sam's personal investment, YCombinator has always stated they invest more in teams than ideas; do you have data to suggest a better approach for picking winners?
Of course, but a great team can adapt their product to meet the market demands and will execute well. A poor team, on the other hand, likely cannot come up with a great product or, if they can, will execute poorly. After all, if they have a great product and execute well then, by definition, they're a great team.