This seems like a misprint. It says in the clarifications that the villain "retreats" back to the streets. The sewer space is always back. It also makes more since flavor-wise that subterranean creatures would retreat underground.
The issue here is the fact that in the Streets, a villain might stick around for awhile. If we assume Burrow refers to the Sewers, then the next villain is going to push it out, making it much tougher to deal with, especially in the case of the 7. Given that one of the villains also refers to the Streets (moving to the bridge) I think they got it correct.