Couple of things after reading the chapter...
1 - After seeing this hit from Coby, it is implied that it is a power that he didnt have before and emphasis is on haki.
However, not a single known form of Armament is capable of doing this. It looks superior even to Garp's aCoC punches.
The destruction range is way different from ID CoA but there was no CoC SFX nor the aCoC-characteristic coating trail.
2 - There was no off-paneled fight between Garp and Kuzan, there is literally no time for it. Garp was laughing as he was falling from the sky, Kuzan nailed him with Ice Spear and that's all. Garp is still laughing and finishing conversation with the ship where young trio just landed.
3 - Garp isnt dead, of course. He may not even be frozen and preserved but just incapacitated due to the ice spear immobilizing him.
B-but, CoC user can break out of it! Sure, when healthy and fresh but Garp is heavily wounded and exhausted.
In the end, the legendary hero that fought with Roger to the death countless times, was defeated by 1 critical wound from Shiryu and two hits from Kuzan, among which one is the final immobilizing spear.
4 - Coby is on a fast lane to be more than Garp ever was. Expected for someone who trained far more than Garp.
As soon as Oda starts handing out haki upgrades to Coby, he will surpass Garp in no time. The moment he gets aCoC, if this wasnt it, he can automatically fight anyone in the world. Winning is entirely different matter as we saw from Luffy and Yamato but fighting "equally" is one upgrade away from being a scrub.
Glad to be back at Egghead, hopefully, Oda doesnt waste more time on the unnecessary running in circle and gets to the big fights fast.