TCP is stateful by definition, and that is why it supports robust error correction and reliable streams of data. UDP is its stateless counterpart.

> bonding, ... usually required application level code

No, it doesn't. https://en.wikipedia.org/wiki/Link_aggregation

> underlying transport layers

TCP is in the transport layer! The layers which you name are lower, such as Layer 2 (Data Link)!

MTCP, being in the transport layer, is working at a higher level of abstraction than bonding or link aggregation. There is no need for application-layer support, but it is possible, and easier, with MTCP than with the Layer 2 solutions, because MTCP is in the transport layer. That is why Go was able to activate it, and the POC uses it, because MTCP controls can be exposed at that level.