Legally in regards to what? A published regulation? You know that they wrote the requirements and are perfectly free to change it right? Agency law is its own separate legal subject.
It's already been covered as to why this wouldn't legally stand up. Of course anything can happen in front of a judge, but "technically" it didn't stand a chance. Or do you think the most corrupt and anti-gun administration we've seen just backed down because they had a sudden change of heart...