Your link to Online Metals was for a brass tube, but that may well be more durable than a tempered ALU alloy.
Using a hub or even two hubs at the base of the these and similar chairs to support the seat appears to be an invitation to fracture. Putting all that pressure on the tubing below and above the hub(s) is too much, IMO. No surprise that REI marketed a repair tube for their model. A good idea for a tent pole section, but not sure it is much of a remedy for a chair that must be lugged home and repaired, and may fracture again due to the hub design.
Should that happen, suggest looking for an alternative in the classic canoe chair design, such as this MYOG chair: https://backpackinglight.com/forums/topic/81152/
The canoe designs tend to put the body closer to the ground, but have found that to be an asset, as it is better for stretching the legs out. The design also allows for swiveling about on the chair with much less risk of breakage. The weak point is at the rails supporting the seat, which were reinforced with carbon tube inserted inside the alloy tube.