Under the hood, Electrum derives a "master private key" (m) from the seed and derive external and internal chains at (m/0 and m/1) for receiving and change address parent extended keys.
Then, the addresses which at (m/0/0~19 and m/1/0~9) for the initial 20 receiving and 10 change addresses.
This is only the case for legacy wallets. Electrum uses m/0' (rather than just m) for single-sig segwit wallets, which has obviously been the default wallet type for some time.Then, the addresses which at (m/0/0~19 and m/1/0~9) for the initial 20 receiving and 10 change addresses.
So they instead used the hardened child extended private key as SegWit master private key.
They probably just stick to calling it "master key" instead of "extended key". (well, that caused some confusion)