How would the real extended public key become known?
If you have enough keys, and you know, that they are all coming from the same wallet, then you can try to retrieve the master public key, and later, the master private key. However, it seems to be not this one, which was shared, because it contains only the key to puzzle 66 (not to all puzzles).
Why zpub and why BIP84 key derivation?
Because it is written in this particular key. But it does not mean, that it is the same, which was used in the puzzle. It can only be used as a hint, to determine, which tools were used by the author of this post.
To my knowledge BIP84 derivation is for P2WPKH, ie. native Segwit addresses which aren't used in this puzzle.
Well, Bitcoin Core can sometimes give you P2PKH, and sometimes P2WPKH. It is possible, that a single master key, will give you different address types in different wallets. For example, you can see P2WPKH in Bitcoin Core, and iancoleman tool can give you P2PKH instead.
So what's the purpose of a half-baked extended public key when it's incomplete?
Some wallets can only load descriptors, or master keys, and in this case, you can write a single key in a "master key format".
So, to sum up: I thought that the author of the post somehow got the master key, but it seems to contain only that single key, but just written in yet another format, to be loadable by some specific wallets.
