It's possible to use kernel facilities to route flows to the right application or stack, and then use that stack to process individual connections.

For example, many network cards support creating multiple virtual network devices, and you could hand a whole such device to a userspace process, but only let it see traffic intended for that device.

Or, finer-grained, you could use a filter to steer certain packet flows to a process, while handling others in the kernel.

I do absolutely think there's value in having native QUIC in the kernel, but then, I also think there's value in having native TLS in the kernel; doesn't mean you always want to use it.