I appreciate your dedication to Metagross!
The defender tier lists will be updated soon to account for the CP rebalance. The Meta article was an aside that Ryan took while working on that, and it is a very interesting take on the "meta." It's also relevant in a post and pre Meta CD (with it returning next weekend) world where that's just a common attacker right now in everyone's collections. Expect an update to the Def tier list soon!
As for the ATL, I personally was a fan of Meta at Tier 1... but that was when Kyogre and Tyranitar were also considered Tier 1. We've since broken up the category to include a 1.5 and I believe Meta correctly finds a home there.
There's other things to consider about Machamp. 1- The CP rebalance was very good to him. 2- He remains a huge tool for lots of Tier 4 Raids. Meta is the top counter for Togetic but that is already gone. Machamp is the top counter for Aggron, Absol, Houndoom, Ttar, Ursaring, (arguably) Walrein, and probably more I'm missing. Fighting has amazing type coverage.
Meta is now a better counter for the 2 Regis it's SE against but even if they do return, Machamp can still knock them out faster in Cloudy where Meta is almost never getting boosted.
I do think Metagross is very, very good, and you should definitely Maxis one or two. It's also an amazing generalist. For my money, it might be the best in Tier 1.5. The Tier 1 attackers, though, are just so incredibly useful that you can justify maxing entire teams of them, something I can't personally say about Meta.