Design flaw may be too strong. There's no perfect design for seat clusters. The traditional way of using pinch bolt ears hat are part of the lug or frame (as on a typical steel frame) has the drawback that if the ears fail as they sometimes do, the frame is toast. Also this doesn't work well on welded aluminum frames, so the alternative of an extended seat tube with a collar is pretty much SOP for aluminum frames.

It isn't perfect because it's vulnerable to seat post flex under heavy loads.

One would think that the post would buttress the outer tube (two tubes are better than one) but this isn't always the case. Differences in flex characteristics can cause a stress riser at the weld leading to your problem. However this isn't a problem in 99.9% or more of frames of this type.

So there's no perfect solution and it's a matter of balancing the benefits and drawbacks of either design. Given your weight, I'd shift the balance in favor of zero extension above the top tube, and/or reinforce the post in the area where it enters the frame.

Stated wheight limits can only be taken as coarse guidelines, because there are significant variables, such as [pavement quality, tire/wheel selection, and riding style. I agree that this isn't good news for heavy riders, but you have to understand that designers design for their intended audience. If bikes had to be built for heavy riders then they would be overbuilt for the vast majority of riders.

I understand that heavy riders are handicapped it their choice of bicycles, but those are the breaks of the game.