They probably just stick to calling it "master key" instead of "extended key". (well, that caused some confusion)
Yeah, it's a bugbear of mine and something that pretty much all wallets (and most users) do. The term "master private key" should be reserved for the key at path m. Anything at a lower level than that is an "extended private key". The "extended private key" unique to a specific wallet is the "account extended private key".
In the case of Electrum segwit wallets, then m is the master private key and m/0' is the account extended private key.