It's interesting that you call this having "very little to do with" the success. It's a philosophical argument, I guess. If there's boulder at the top of a hill and I give it a little nudge so it rolls all the way to the bottom - did I have very little to do with the boulder rolling down, since gravity did most of the work? Or did I play the most important role?
Even if it's true that selection bias is huge and the best qualified people going into a bootcamp are the best performers coming out, that doesn't necessarily mean the bootcamp wasn't useful. These people could have otherwise gone their whole lives always being one small nudge away from becoming a great programmer, but would have never realized it without the right conditions. If a bootcamp simply creates those conditions for them to make it happen for themselves, that still seems important.
Even if it's true that selection bias is huge and the best qualified people going into a bootcamp are the best performers coming out, that doesn't necessarily mean the bootcamp wasn't useful. These people could have otherwise gone their whole lives always being one small nudge away from becoming a great programmer, but would have never realized it without the right conditions. If a bootcamp simply creates those conditions for them to make it happen for themselves, that still seems important.