They probably designed their tolerances assuming perfect welds and then didn't have them.
This sounds like terrible engineering practice.
Is it even possible to discuss anything Elon Musk anymore without 100% praise of his engineering prowess? Every single comment gets down-voted into oblivion, with very few replies. And, due to the "cool off", this prevents any balanced discourse.
It is more an issue of quality control and inspection routines.
So it has to be as thin and fragile as possible but no thinner. That makes it very hard to get right, and the welds have to be done to very fine tolerances on thin material, you could say it is rocket science.
I understand. But the stresses that materials and welds can withstand are not unknown variables. We're talking about a pressurized tank here. You aren't going to build a rocket (designed to carry people) that has tolerances so tight that it's touch and go whether it explodes as soon as it's pressurized.
Isn't weld strength something engineers would know/understand in advance, though? How would it come as a surprise that the welds would fail under pressure?
It seems strange to me that Musk could be spending so much time working on Starship, and somehow a "badly designed, badly built, and badly checked" rocket makes it to the platform and explodes.