"As someone a bit dumb but interested, would this be possible if HTTP2 was being used?"
Yes it would still be possible. HTTP/2 only changes the wire format of the HTTP layer messages and makes TLSv1.2 the minimal version. The SSL/TLS encryption protocol is where the attack is happening. They can use all the trusted CA trickery to intercept connections of any type (email, ssh, even VPN, ... whatever uses SSL/TLS). The downgrade to SSLv2/v3 on the server connection would not be possible in HTTP/2, but that is not a necessary part of the hijack anyway.