If I remember correctly that is a bolt welded to the tower to make a stud. That is a bad spot to be in as far as repairs go. If it were me. I would probably take the strut brace out and cut the studs off at the tower. Then drill down through the head that IIRC is welded to the tower. For the sake of not taking the whole car apart to cut off or grind the old heads off inside the strut tower I would just run a new bolt through the head of the old bolt if that makes sense. Do not try and rethread what remains of the stud. There won't be anything left of it.
This is just my random thought. Someone may come along with a much better idea.
If that stud is in fact a bolt welded into the tower upside down you absolutely could cut the stud off and then drill and tap the head from the top side of the car. You could then loctite in a new stud for the brace or just switch it over to bolts.
Pretty sure mr black is right on the money. It probably is a "bolt" with a circular, flat head that is welded to the sheet metal. It will be hard to get at it from inside the wheel house even with the strut removed because the stamped steel cup that surrounds the strut mount is in there getting in your way.
i would tack weld the new bolt in if you can. It's going to be a pain dealing with R&R'ing the strut tower brace from here on out otherwise.
thank you for the additional info nothubertj. i believe i am going to remove the tire/wheel to look around in the well, to see exactly what I will be drilling/tapping into, but I am definitely going with the from-the-top approach. I like the idea of working it with a bolt rather than a nut on that post anyway.