Can anyone tell me what the difference is between the cold card and other air gapped devices [emphasis mine].
Besides the fact that indeed, hardware wallets all have the same basic purpose and solve it in a mostly equal way, there are a few differences like price, open source code, open hardware, codebase, secure chip, overall physical form factor and features, coin selection and - finally - air-gap.
I would like to point out that the devices you already own are not airgapped by the typical definition. Since they connect (for power and communication) to your PC's USB port, there is a slight potential attack surface through that interface.
Meanwhile, an airgapped hardware wallet can use a memory card (like in ColdCard) or a QR code (like in
Passport) to communicate without a potentially compromised wired link. In my personal opinion, an airgap is quite valid to have, since the very purpose of a hardware wallet is to guard your coins from a potentially infected computer. With this risk assessment though, you should also assume the malware could access USB and mess with that.