Dec 10, 2018

Ah, Maxwell clarified additionally eventually. What I understand him to mean is that, conceptually P2WPKH is P2PKH just with the signature in the witness instead. P2WPKH's nature is such that, when you endorse spending a P2WPKH output, then the pubkey script regarded is, conceptually, DUP HASH160 hash EQUALVERIFY CHECKSIG, and for this reason that's what an endorsement in a witness should sign. (He then goes on to make a separate point that the native P2WPKH pubkey script can be seen as "compressed" in the sense that OP_0 PUSHDATA [20B] is 22 bytes while DUP HASH160 hash EQUALVERIFY CHECKSIG is 25 bytes.) I guess that brought final clarification to it.