by forcing out the rage fist and cycling fighting resists/helmets whilst doing damage back you’re weakening ape into KO range. And eventually your offensive or tanky Pokémon can KO it. This isn’t a new concept.
So on stall, run a (special) pokemon that can infinitely switch into ape's drain punches and can OHKO it. I don't think you realize how narrow this list of pokemon is, because anything less than an OHKO and he's just switching out, coming in on one of the many status moves or weak attacking moves you have, and Drain Punching to heal back. Gholdengo? 40-47% with SPECS with Tera Water. Specs Pult? Not even a guaranteed 2HKO with any of his moves. Even Iron Moth's Psychic or Dazzling Gleam isn't enough to 2HKO WHILE NOT TERA'd. That's not even to mention adding something like this makes your team, as a stall team, significantly worse. What should you replace with an offensive pokemon just to make sure you don't auto-lose to Ape? Either Clodsire or Blissey, and now you can't handle powerful unboosted special attacks or can't deal with setup? Dozo, so you insta-lose to any boosting physical threat? Either Pex or Alo, so now you can't deal with Garg anymore? Oh, and if you're relying on attacking moves on any of your non-offensive pokemon to beat any threats, like Liquidation on Dozo, you can forget it. Ape just switched in and you gave him a 100 bp ghost moves that chances are that your offensive pokemon can't deal with. Quoting from the ban reasoning, "This is a very common way Annihilape spirals out of control against teams that don't overprepare for it, and sometimes even those can be very much overwhelmed."
Oh, and this is what happens in the tangrowth matchup lol.
Annihilape uses Bulk Up!
Tangrowth uses Giga Drain!
Annihilape lost 15-19% of its health!
Annihilape now 2HKOs you with Rage Fist (assuming, as you do, that leftovers have been knocked already, which is far from guaranteed in gen 9)
OR
Annihilape uses Bulk Up!
Tangrowth uses Power Whip!
Annihilape lost 25-29% of its health!
Annihilape uses Bulk Up!
Tangrowth uses Power Whip!
Annihilape lost 18-22% of its health!
Annihilape uses Bulk Up!
Tangrowth uses Power Whip!
Annihilape lost 14-17% of its health!
Annihilape uses Drain Punch!
Annihilape heals 16-19% of its health!
It literally stalls out Power Whip with Drain Punch, assuming all Power Whips hit lmao