If I insert it into a computer then the malware already on that computer can infiltrate the wallet right?
I don't know who said that hardware wallets are unhackable but any electronic device can potentially be hacked at some point.
If you are connecting your hardware wallet with usb cable to your computer you are not exposing your seed words or private key and there is no direct connection with internet.
There are however several
attack vectors for Hardware Wallets, someone can extract your seed words if you don't have passphrase, someone can get your paper with seed words, or you can make a mistake and enter seed words in some phishing website.
Hardware wallets are generally safer than hot wallets but only if they are open source and without some hidden code,
and like we saw in recent example when FBI was running and selling fake secure encrypted phones ANØM for 3 years and busted some criminals worldwide.
Lesson is - never trust closed source ''secure devices''.