QUIC explicitly mentions that it is vulnerable to amplification attacks in the RFC. It suggests sending an extra packet to mitigate, at which point I believe the advantage of QUIC is lost as far as establishing connections is concerned.
I believe you're referring to the stateless retry mechanism? It's designed to be used only when the server is actually under attack (either as an amplification vector or exhaustion of its own connection capacity). The idea is that the server establishes some threshold for pending QUIC connections, and only if that is exceeded does it start requiring clients to complete the extra stateless, SYN-cookie style roundtrip to validate their source address. So it maintains the advantage of one less roundtrip than TCP+TLS unless the server is receiving large amounts of connections that are not progressing in a timely manner.