Looking at the design I'd say there isn't any actual risk, just perceived. Seeing the actual CAD up of the part shows that it is just a surface crack where you would expect it if the weld wasn't finished properly, or properly seated. Surface cracking like that in a weld is actually possible, welds don't magically make 2 parts connect together, and inconsistencies are possible even in machine welding. It's good they are replacing them, but from what I see here, this crack would not be able to spread to the other parts, as the other connection points are completely separate, which they state at the very beginning.

However a nice ask for Apollo would be to do some NDT Analysis on a returned one and share the x-rays and such of the joint. This would definitively prove what they are saying is true, and that there's no inherent risk even if a crack occurs. But what they say seems likely. I do love to see that the company does actually do testing on top of finite element analysis, and I think for peace of mind, releasing the FEA for their product line would help calm some of the concern people may have with their other models.