definitely an open discussion
im approaching this mostly from an alpine climbing perspective (long days on your feet, approaches with gear, etc) and also from a fast hiking/trail running perspective although I would not qualify myself as a trail runner.
It makes total sense that weighted uphill carry would help, but then again, a lot of things have made sense in the sports science unworld untill they were totally debunked/proven wrong.
Walking uphill with a pack does create greater time under tension VS running. In both cases there are decent forces applied to the ground but the time of the contraction is what changes. Reflecting on it, I would think the uphill carry would MAYBE be more beneficial VS the gym based ME. Gym based ME doesnt work strength because the loads are so low, doesnt work endurance because the volume is so low, sort of a middle ground here. Outdoor ME workout really reminds me of tempo efforts but very specific to uphill carry capacity/sports which I keep questioning myself why a good endurance volume + max strength in the gym wouldnt acheive the same thing.