If you improve ductility surely you destroy the heat treatment? Anyway, you could be right. They claim welding it should not reduce it to 631 according to this graph:



853 with no filler (obviously not how you would actually make a bike) is stronger all the way between the weld and the base metal, implying the air hardening more than makes up for any loss of heat treatment, and the 853 line stays a good 300MPa above the 631 line at all times. The reason they show a fusion weld on this graph is because the filler metal is weaker than the 853 to start with (so you would get the blue line in practice if you used ER80 filler).

They do say this graph is not based on actual testing but "on current knowledge and volume fraction data".

So maybe you can heat it right up to red hot, bend it, and it just air hardens back to exactly its original heat-treated strength. This seems a bit too good to be true though.