I mean, it's less a lie than an estimate. We don't have exact numbers, so it was a range. And I'd have to double check, but that was also the range for just Jewish victims, counting the Roma, homosexuals, Russians, disabled, political opponents and general undesirables, the numbers the Nazi's were said to have slain is roughly double that figure.
Stalin and Mao also have higher body counts, but they also were less purposeful in how they went around with things, so most people don't hold it as comparable. Still completely barbaric, mind you.
But, let's say, for the sake of argument, that number is too high. Estimates were iffy, it's closer to 4 million, or even 2 million. So what? When does the targeted eradication of an ethnic group become fine? I'd be hard pressed to find a reasonable situation where it ever would be fine.
And of course, the only time I ever hear this sort of argument is "I wish it was 6 million" or "Should have been more than 6 million" or other terrible shit where the speaker/writer reveals themselves to be all sorts of shitty.