> Exploitability over the internet is unlikely, most ISP routers drop packets with ipoptions fields.
That doesn't track. Apple iPhones are a major user of MPTCP and they report the majority of requests supporting MPTCP, IIRC 70+%.
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.
MPTCP uses TCP options fields, the vulnerability is in IP options.