If I interpret their argument correctly, they are not talking about raising VC immediately after YC. Their argument is that, at some point down the line, if you try to do a VC round then the sole VC that participated in your seed round has undue control of your first VC round.

I think that's a misunderstanding of the YC / Sequoia relationship. YC raises from Sequoia just like a VC raises from their limited partners. What you're describing sounds more like if YC were doing a syndicate seed investment with Sequoia, which isn't the way the deal works.

This clarification makes a lot of sense to me.