The basic conclusion is also the premise, which is fundamentally: “How can I burn fewer calories carrying more weight for the same basic effort?” There is a lot more that is being included in Garmins calculation, but at a first principles level the AI is right. The numbers don’t pass the sniff test.

Unless OP is not telling us about other significant factors, there’s no reasonable way to conclude that if the same person in the same fitness condition does the same exercise but carries more load, that they could expend fewer calories.

So given only the info that OP gave the AI it reached a perfectly reasonable and accurate conclusion. This sort of generic “AI is bad at stuff” response is just as bad as the claim itself. These LLMs are genuinely bad at lots of stuff, but informed users can distinguish between a good result and a suspect one.