White supremacist gangs are generally privileged enough to afford regular guns. Crackdowns on gun possession have overwhelmingly targeted and been selectively enforced against black and other non-white communities and left wing groups, whereas Kyle Rittenhouse shot three people at a BLM rally then walked right past the cops who didn't even intercept him, then he was acquitted. The only BLM protestor to shoot someone was basically assassinated by the feds.
There are plenty of problems with guns in the US and I think there are a few regulations around purchasing them that make sense, but this is virtually a non-issue. Printing guns is so time and resource intensive that unless you are arming a militia in a place where the general population has very limited access to guns, it's not really going to change much.
The one other thing you can do with 3d printing is make full-auto guns and modifications, and those are scary but not really important. Full auto has limited tactical use in squads in pitched warfare. Semi auto will work for 99% of cases, and if the need for full auto is coming up a lot, you're probably in a position to use your gun-to-get-a-gun. We're talking about situations where the law has already broken down, like Myanmar.
Now I can understand why the state would be scared of the tech, but I don't see it helping white supremacists. They already have all the help they need.