I could see where Traeger makes an argument that their software for controlling auger feed so as to maintain the owners temperature setting is based on the properties of their patented pellets. Their energy density, their burn rate, etc. When designing that control loop software, you'd have to make some assumption about the fuel coming into the burn chamber...
It's interesting that other brands of pellets work just as well in terms of final outcomes. I guess they all behave the same in terms of burning properties, more or less.
From a marketing point of view, though, if I were Traeger, it would be a soft suggestion, because it would be unappealing (to me) to purchase a unit that demanded one exact brand of pellets. For Traeger likely their business plan is based on the old razor and razor blade business model, and they want you to keep buying pellets forever.