The rule in effect is rather narrow and doesn't actually ban home manufacturing. All the elements of a kit are still accessible and legal.
The only difference is that all the parts to finish the controlled part can't be sold together. So like you could by the 80% from one shop online and the jig from a different shop online. All the other parts wouldn't be affected in general, maybe an issue if sold with the 80%. And there are also other ways to do home manufacturing that would be completely unaffected but the rule.
Also the case isn't done. The order is a temporary stay where the court is asking the ATF lawyers to explain things.
ATF has some rulemaking authority. For example, when Congress says "no commercial firearms sales without a licence", well, at what point between raw material and finished product is that object a firearm? ATF says it's at 80% completion.
I get what you're saying and the question is not out of line.
It makes a bit more sense if you think of the ATF as the FDA but for guns. They're supposed to be the subject matter expert. So it's not completely out of line for the court to ask them to explain matters relating to guns.
Sure, but legal processes aren't quick. I would assume they're trying to be thorough in the process. Probably not a great sign they letting the rule stay but in theory it doesn't actually do much. 80% manufacturers can still sell products. I can still buy an 80% or a 3d printer.
The real thing would be to just get a ruling to limit how they can change law through changing definitions. Same reason slapping down the bump stock was needed.
Also the issue there ruling on probably won't actually be a 2a thing but about the rule making effectively side stepping the legislative proceess.