A bonded/teamed interface usually load balanced based on a flow. Thus, over a single TCP flow you will only use one of the interfaces.

On top of that MPTCP also offers advantages when it comes to handover in mobile networks: if your IP address changes for some reason (e.g., switching from Wifi to mobile), existing connections won't be interrupted.

Bonded interfaces have the same IP (iirc), MPTCP can send traffic over multiple IPs, eg over both wifi and 4g, for better roaming. Especially useful when moving around and entering/leaving different wifi networks, all without dropping the TCP connection.

Suppose the sender has a single interface, and it's the receiver that has multiple interfaces. With MPTCP, the sender will learn that there are multiple paths available, and it'll balance its load adaptively over those paths.

I don't know enough about bonded interfaces, but I don't see how they'd help in this scenario. This scenario is natural with mobile devices, which is probably why Apple uses MPTCP.

It's more related to multihoming than link aggregation.