It's just noted in the FAQ that there's no other option to import into an existing HD wallet so it's suggested to use sweep instead;
But just below it, there's an instruction on how to create an imported wallet to import the private keys instead of sweep.
Your link w/ highlight: electrum.readthedocs.io/en/latest/faq.html#can-i-import-private-keys-from-other-bitcoin-clients
As an addition to the other replies, here's what they're talking about:
In not-so-old versions before v2.0, they included a warning about the dangers of importing a private key to an HD wallet;
v1.8.1 screenshot:
Then after 2.0, although the menu is still visible, it's grayed-out if the wallet isn't "[imported]";
v2.0 screenshot: