Wireguard is opaque about the independent streams in its connection. So, while they both can encapsulate multiple concurrent streams in one connection, QUIC can do things like mitigate Head-of-Line Blocking and manage encryption at the transport layer. It also uses a connection ID on these substreams which helps make transitioning across network changes seamless.

If you set up multiple TCP connections over Wireguard, there is no head-of-line blocking either. And Wireguard also transitions across network changes.

In fact, it's one of the main reasons I use Wireguard. I can transition between mobile network and wifi without any of the applications noticing.