I kinda thought the same thing about flash-drive viruses, and why Bruce wasn't using CDs/DVDs instead. Then I realized if he was really serious, he wouldn't say what he's really using, and he'd have a USB honeypot plugged into his network-facing computer.
Note that he didn't say what software he's running on the new air-gapped computer. The difference between locking down one air-gapped PC running only software required for encryption and locking down the entire network of PCs running the wide variety of software required to do everything everyone needs to do on the SIPRnet is huge.
