They have never adjudicated a completed and certified POTUS election and never will.
So what. They never had to and they never were asked to. That doesn't mean that they can't.
Would anyone in the US respect the power of the Federal Courts (none people, ultimately) to overturn a POTUS election, even for cause?
Months after congress has certified it?
Sure, they could have their hearings and trials but they have no clear constitutional authority to rule in this matter.
Any sane POTUS would simply say no, and would receive backing by congress to raise sufficient forces to say no.
Likely with the support of the majority of the American people.