Did they even bother to miter those tubes though? It would be a different bike after repair if they didn't. But the way that the lugs are so long onto the top and down tubes makes me believe they just cut them square.
I built a 531 frame back in the '70s and I like it more than I think I should. There is a lot better tubing available nowadays, but if some 531 fell into my hands for a decent price I would probably build another one. I don't see anything wrong with a 531 mountain bike. And of course, the 531 didn't break on the OP's bike anyway