> The body shop owner told me that the epoxy is actually stronger than the factory weld.
Often this is because the special high strength steels used in vehicles today depend on proper heat treating to attain their strength, and welding can compromise this. Many OEMs even specify panel bonding for repairing particular crash-critical parts of vehicles now because of this.
1. because UHSS is sensitive to heat, and robots are much more accurate in how they heat than Jimmy with a tig torch, and they were programmed by a process engineer, where as Jimmy welds until 'it looks good'.
2. welding may compromise anti-corrosive treatments on the inside of inaccessible cavities, which can lead to corrosion issues
e.g. https://rts.i-car.com/crn-24.html
A crappy shop will certainly just weld panels in without any regard for materials engineering, but it results in a crappy repair.