The fact Electrum Mobile can do it is really odd... either neither should or the Electrum Desktop is missing the feature?
It's not possible in the mobile Android version either.
What he described is the same as the Desktop versions new wallet menu "
Import Bitcoin addresses or private keys" which will create a separate [imported] wallet.
It'll not be added to any existing HD wallets.
That wallet type can accept additional private keys since it's not backed-up by a seed phrase.