(author here) I didn't have a chance to test how the VPN works, but even if they were using MPTCP the switch wouldn't necessarily be seamless.

Especially how they are using it now - which hands over a Wi-Fi call (which is data) - to a cellular voice call (not data). It's a subtle point, but calls over cellular are not data, so they are not using TCP/UDP/MPTCP/etc.