You're absolutely right. This draft tries to pull the wool over the reader's eyes by saying "UDP allows a single packet response from an application. TCP and S

You're absolutely right. This draft tries to pull the wool over the reader's eyes by saying "UDP allows a single packet response from an application. TCP and SCTP operate differently. TCP has a three-way handshake..." In reality, TCP is no different from UDP and will respond to a SYN packet with a spoofed source address. The reason why that's not a problem is because the response provides no amplification. So you just design your UDP-based protocol to never respond to an unauthenticated packet with a larger response and it's fine. Indeed, that's how modern UDP-based protocols are designed, since we understand this now. The only problems are with old protocols which were designed before anyone knew better.