I emailed Dr. Peck for clarification. I probably should have done that prior to posting.
This is copied and pasted from his response to my question about adding acid to sugar bricks.
"Acid in a sugar brick, because there is no water in the mix, does not allow HMF to form from fructose (and doesn't even allow the acid to invert the sugar from sucrose to fructose, which also requires it to be floating suspended in water.) It is probably unnecessary, but there is no reason to believe that the acidified bricks will harm the bees. The smell of the vinegar may even help them find the bricks in the hive."