Downvotes are allowed to express disagreemnt.
Actually there are no guidelines about voting whatsoever.
However arguably it's a bit rude to ding people simply for being (mildly) misinformed.
And the guidelines do ask us not to be rude.
According to pg: https://news.ycombinator.com/item?id=392347
Doesn't mean it's a good idea.
There was no reason to downvote it.