On second thoughts, I just read the below. Basically the packet size should include the headers… So therefore the MTU minimum should indeed be 1280.
14. Datagram Size
A UDP datagram can include one or more QUIC packets. The datagram size refers to the total UDP payload size of a single UDP datagram carrying QUIC packets. The datagram size includes one or more QUIC packet headers and protected payloads, but not the UDP or IP headers.
The maximum datagram size is defined as the largest size of UDP payload that can be sent across a network path using a single UDP datagram. QUIC MUST NOT be used if the network path cannot support a maximum datagram size of at least 1200 bytes.QUIC assumes a minimum IP packet size of at least 1280 bytes. This is the IPv6 minimum size [IPv6] and is also supported by most modern IPv4 networks. Assuming the minimum IP header size of 40 bytes for IPv6 and 20 bytes for IPv4 and a UDP header size of 8 bytes, this results in a maximum datagram size of 1232 bytes for IPv6 and 1252 bytes for IPv4. Thus, modern IPv4 and all IPv6 network paths are expected to be able to support QUIC.