Garou’s numbers are 2 ZettaFoe, whereas 5th Form Cooler without any further amps was at 216 ZettaFoe.
The Supernova forced SSJ Goku at 540 ZettaFoe (not accounting for the KHH’s amp) to the brink.
By the time of Return of Cooler, Mecha Frieza showed up and was dispatched by Trunks, who SSJ Goku and Vegeta were superior to and scale to the 1 YottaFoe range (as Trunks when he Time Traveled was considered “about as strong as Gohan”).
That’s about a 500x-ish gap? Bare minimum? Combine that with the jump of power Supernova grants, as well as the extra Coolers pooling their power into one move, and you’re barking up a significant number of extra extra zeroes. (For reference, in the Lord Slug film, Combining Ki with just Piccolo while both were on death’s door gave Goku the ability to handle the 100x Kaioken. Combining Ki with all of his friends in the Broly movie gave Goku the ability to defeat Broly, who could destroy 1/4th the Living Universe, which {unfortunately} is accepted as infinite. Even ignoring that, you’re going from galactic to countless galaxies.)
The hypothetical Cooler Group Supernova would absolutely be enough to wipe out Garou. Even if we wipe out the 500x initial gap and immediately have him his own YottaFoe high end on his profile so they’re fundamentally equal at round start. (Because the jump in power Ki Sharing gives is very wack and massive for some reason).
Note: The amount of power Ki-Sharing gives isn’t consistent—Goku only got a 100x or so boost in Lord Slug, but in the Broly movie got a waaay bigger jump. Baby absorbed the energy of the Saiyans and Tuffles and only got SSJ2 and 3 level jumps. SSJ4 Goku absorbed his Family’s Ki and got only a little over 2x power (though to be far, they also took him from zero Ki to that level, so a lot of Ki is being spent to just refill his power, which is absurdly into 2-C). Even in the Shadow Dragons Saga where Goku is going “to the limit” it’s vaguely above 10x (same thing here, though. Was at zero, now at this new above 10x amp). And the worst offense is in the Anime Namek Saga, where sharing Ki between Piccolo, Gohan, and Krillin didn’t do anything at all.
The only reason why it’s such a substantial factor here is because Cooler literally has a hive mind of hundreds of himself that makes it more likely than not (basically a guarantee) he’ll get an upper end result, not a lower end one, which puts Garou in hot water, because upper end results are absurd jumps here.
So Cooler was a 20, and SSJ Goku is a 50.
An at best slightly above x2 power amp really isn't going to be that helpful.
Cooler was above a 20x direct amp combined with a 3x PL amp (which act as far greater stat jumps than the numbers imply, unless specifically caused by a transformation that works linearly). Same goes for SSJ Goku’s S.KHH.
