Gopnik screwed up the math
It may be because it was in the dream sequence, but when he wrote the uncertainty in momentum (Δp) he messed it up. He wrote
Δp = sqrt(<p>² - <p>²),
which effectively is Δp = 0. He should've written
Δp = sqrt(<p²> - <p>²).
I don't know if this was an oversight in the editing or if it was a wry bit of irony, considering that Gopnik says you can't understand the physics without knowing the math.