I assume this is simply a translation error. Your transaction should not be "partially signed" online. Indeed, transactions can only be partially signed if you are using a multi-sig set up. With a standard single-sig cold wallet, the only thing that happens on your online machine is you create an unsigned transaction. That unsigned transaction is moved to your cold device to be signed, and then moved back again to be broadcast.

If you simply install Electrum on an internet connected device and use it as a hot wallet, then yes, it will not be as secure as a (good) hardware wallet. But you can also use Electrum as a cold wallet. What this means is that Electrum is installed on a computer which is permanently disconnected (airgapped) from the internet, meaning the device can never download malware and never be attacked via the internet since it is never connected to the internet. This airgapped computer stores your private keys, and your private keys never leave this airgapped computer so are never at risk of being exposed to the internet. You create unsigned transactions on your usual internet connected computer, move the unsigned transaction via a USB drive or QR code to your airgapped computer to be signed with your private keys, and then move the signed transaction back to your usual computer to be broadcast to the network.