I would be curious about this as well. The only attack that I can think of that he might be referring to is the server publishing bad keys for a particular clie

I would be curious about this as well. The only attack that I can think of that he might be referring to is the server publishing bad keys for a particular client while that client is offline. This equivocation would be caught fairly promptly though, and I think that eventual proof-of-equivocation is better than forcing users to manually verify keys.