This rule doesn't apply if you've taken the necessary precautions. For example, if you've verified the binaries of Electrum on a freshly installed open-source OS, on a machine that you've removed things like WiFi antennas, then you're fine. Turn off the lights, close the curtains, generate your seed phrase, write it down on a paper, copy the master public key, and you're safe.

I don't use Bitcoin Core as a wallet. There's no reason. Stick with Electrum that's lightweight and don't make things more complicated than they already are.