Two things need to be explained.

First, the innkeeper is referring to the fact that when Winslow and Eddie wake up from their magic-induced stupor, they will of course realize something's up. You do not normally wake up standing straight up with your last memory being the vague image of a girl outlined by a bright flash of light. Matt and Alex's answer to this, that is, physically knocking the two of them out, allows for at least a plausible excuse on the part of the innkeeper while they get away. It's not a good excuse, but it's an excuse, and that's more than what they started with.

Second, it should be noted that Alex is, indeed, a sorceress, not a pugilist. Thus, one may wonder how she managed to floor a Landis Troop with one punch, with her non-dominant arm, no less. This can easily be answered by remembering Winslow was completely incapacitated at that point. Without any sort of reflexes or ability to react whatsoever, even a swing from a mage can knock a person on the floor, moreso when said person is standing and will thus lose balance and fall over from the blow.