I've never said inventory power is the only thing to use for matchmaking, but it should be in there. My main account has been in league 2 since showdown started, and it was around 14k inventory power at that point. This post was talking about a new account I made and is currntly at about 7k. The player I posted about was in league 3.
I guess some of my posts ONLY talk about inventory power, and that is because it's obvious about the other factors that should be weighed in for matchmaking. Inventory power is a part of what each player can build from to get their sp power, as well as a sign of experience of the player(unless it's someone that paid for everything they have). It's just that so many ignore what inventory power is and just blow it off.
And for someone that is putting together a team for league 3 and they go with 4 mechs at 4 4 3 3, and use all legendary endgame mechs, they will have the upper hand over someone with 4 epic mechs in that case. So the higher tier levels means the better mechs that are available.
And, if you look at the person I posted, they can have a very strong team in league 2. They have the mechs, the weapons, and the pilots that are needed. On my 1 1/2 month old account, I have 2 pilots. With mechs I can put together a league 2 squad, but without the pilots, I get owned in all the matches. This is all a numbers game, so you can't look at just the numbers you decide are important. They all have some sort of importance in the game.