There are two types of multipath TCP and they are not at all the same.

What Apple does is, for example, being able to have a connection over WiFi and 5G simultaneously, with different IPs and routes.

The more traditional server side multipath is that you have a network card with 4 ports (eg 4x 25gbit) and connect them to a router, and make those 4 physical ports (paths) behave like a single interface. This helps either load balancing or redundancy.

The latter is what this vulnerability is about.