The 6 million number was a best-efforts estimate at the time. It is not guaranteed to be precisely correct.
(This is one of the very few talking points used by antisemitic asses that might actually be valid.)
(This is one of the very few talking points used by antisemitic asses that might actually be valid.)
Indeed, best-effort estimates for the number of Jews killed range from 4-6M, with similar variance wrt actual cause of death (split among starvation, typhus, gassing, military action, etc.). It's very hard to pin down exact numbers. It's unfortunate that the fog of war isn't explored more often in the topical explanations of WWII most people are exposed to; as you stated, it gives ammunition to historical revisionists.
Doesn't change the atrocity much and it's somewhat consistent with the records (4.5 million known death) and the estimates (up to ten million)
What it does however is it shuts down most attempts of rational discussions and preemptively kill most historians effort to tally the other victims
edit: case in point, downvotes