MPTCP isn't TCP over TCP.

It’s odd

> If MPTCP is not supported, OpenMPTCProuter can also use Multi-link VPN (MLVPN) or Glorytun UDP with multipath support.

It’s unclear to me from a brief look why they use mptcp as a backend for bonding

I don't doubt that it can be a problem in some cases with bad connections but personally I have never had any issues using TCP-based (TLS) VPNs in the 15 years I have been using them, even with MPTCP, which I routinely use to bond several different connections together.