Multipath is different in purpose though, although it does also break up the tuple. It's designed to allow load balancing across multiple addresses/interfaces. You need to explicitly tell the other side "I am also available under the following address". There is definitely some overlap and there are proposals for multipath QUIC which gives you both.
I tend to say the OSI model got it wrong because TCP should live on top of TCP and use its mechanism for ports. That also would have made it easier to launch new protocols with connection and reliability semantics.