How are they going to determine what is HD? Some scholars get labeled denialists because they have different estimates for the number of Jews killed in concentration camps. Seems worrying to have FB moderators determining what's acceptable historical debate and what is wrongthink.
Holocaust denial is banned in Germany (same as swastikas by the way). It works really well. The only people complaining are literally people trying to twist perception of the arguably worst crime in the history of humanity, to make Germany look like a victim. FB is clearly doing the right thing here. Free speech has limits.
Because it's banned, it's impossible to know how well it works.
I'm reminded of Iran's leader claiming they have no gay people. Of course he would think that, because if they were openly gay they would be jailed.