QUIC does not depend on UDP datagrams to be delivered in order. Re-ordering happens on the QUIC layer. Thus, when receiving, the kernel passes a batch (i.e. segmented super datagram) of potentially out-of-order datagrams to the QUIC layer. QUIC reorders them.
Maybe https://blog.cloudflare.com/accelerating-udp-packet-transmis... brings some clarity.
For example GSO might split a 3.5KB data buffer into 4 UDP datagrams: U1, U2, U3, and U4, with U1/U2/U3 being 1KB and U4 being 512B. When U1~4 arrives on the receiving host, how does GRO deal with the different permutations of orderingof the four packets (assuming no loss) and pass them to the QUIC layer? Like if U1/U2/U3/U4 come in the original sending order GRO can batch nicely. But what if they come in the order U1/U4/U3/U2? How does GRO deal with the fact that U4 is shorter?