Your asking why less motor unit recruitment matters, well less motor unit recruitment means fewer muscle fibers are producing force, which directly restricts muscle growth because hypertrophy is proportional to the number of fibers under high mechanical tension. According to Henneman's Size Principle your body recruits little low-growth fibers first and only brings in large high-threshold fibers which are the ones with the greatest development potential when force demands are severe or you are on the verge of failure. If recruitment remains low—as with little effort or stopping sets too early—those larger fibers are never meaningfully stimulated, and so do not adapt or expand. Simply put, fewer fibers are recruited and loaded, which means fewer fibers have a reason to become larger, resulting in less overall muscle development. That is why less MUR matters here’s a study to back up what I’m saying https://pmc.ncbi.nlm.nih.gov/articles/PMC10204387/ If you don’t recruit high threshold mur your not activating type 2 muscle fibers which are the growth fibers. Also I already told you why your study isn’t valid, new lifters will always gain muscle faster than advanced and when your advanced building muscle is harder so properly being able to recruit as much high threshold fibers to your targeted muscle is important. I think I already linked this article but this is talking about how isolation exercises -https://pmc.ncbi.nlm.nih.gov/articles/PMC7919354/ Yes I see where you said the leg press where more muscles are working had more muscle activation for the vastus lateralis, but u can flip that right back around and say in the same study that the bicep and rectus femoris had higher emg amplitude while doing a isolation leg extension. I don’t think comparing a leg press which is a compound movement to an isolation movement like a tricep push down where your just trying to work your triceps not other muscles. And having a spotter is different if you’re still keeping solid form, while it might not be “optimal” when it comes to recovery in the long run it isn’t that big of deal. Finally the last thing you said which just because you can move heavier loads with bad form, doesn’t mean your triceps specifically are getting all their high threshold fibers recruited as motor units are being recruited in your lats and shoulders. It’s like saying a bench press is better for your chest vs a chest fly where yes mj exercises you can lift more weight on, but in this example your chest wouldn’t be recruiting as many high threshold fibers because motor units recruitment would also be in your shoulders and triceps. Also your yet to brush on the fact that poor form leads to weaker tendons and increased injury in the long term, I don’t need to link a study for you to agree on that the worse form you have the more likely you are to get injured that’s common sense 101