Author here. It was fun trying to actually use MPTCP. There aren't many resources on how to actually run it and what to expect. There are many unpolished aspects, like I was able to find (and report) a bug in MacOS (which has been fixed since then).
MPTCP promises a lot, and when it works, it's pretty cool. However, I think that QUIC might out-innovate it. Time will tell.