I too do not know enough to not feel silly for asking this, but I have a similar feeling, and I am not sure if I have misunderstood the past year's worth of popular articles on USB security, the risk posed by the USB in the context of the ceremony, or both. I had gotten the impression that the firmware vulnerabilities of USB flash drives had proven to be inherent, unavoidable, and utterly devastating; that, basically, any given USB drive could become a general-purpose bus-querying hostile computing device, if once plugged into a machine controlled by a capable and motivated adversary.

If that understanding was correctly received, and given the strength of potential motivation involved here, my thought is that the problem presented by the USB drive could only be combated by (a) ensuring that the USB drive was obtained in a random enough fashion that vast numbers would need to be deliberately compromised with this specific target in mind for that approach to be feasible, and (b) ensuring that the USB drive was neither ever unsupervised nor connected to any USB port (except one on a "known-to-be-clean" computer) after the selection occurred.

The problem that I see with those measures is that if compromising the laptop with a compromised USB drive were possible, then the actual security of the process would be purely dependent on the security of measure (b), as established by a single actor at the ceremony: that is, whomever provides the USB with the key-signing request. This last point just by itself would seem to be a degree of risk well beyond the criteria implied by the established protocol.