Basically:
If I am being correct, K selasirby's abosrption consists in trapping who/whatever he swallows in a pocket dimension inside of himself, and gaining powers from those people/things.
The issue is that 96 has dimensional travel, so he can get out, and even i that wasn't the case, he managed to escape from the Emperor's Key more than once, which traps and seals Numbers absorbed by Astral, who can use the powers of said absorbed Numbers. Once 96 got out, even in a much weaker state thean this, he was abel to absorb Astral back. and once he even managed to absorb all the Numbers inside of the EK (which at the time were few though) other than Utopia.
And that's not counting the fatc that 96 in this stae is a Chaos Number, who Astral wasn't capable of absorbing at the time, and almost killed himself, when 96 after a battle entered inside Astral's body to kill him from the inside, only to seal his powers again (not absorbing him).
Oh, and if Kirby absorbed one of 96's monster, the latter could potentially powernull Kirby with cards that counter Numbers, such as Number LockDown, which prevents Number monsters (which Kirby would most likely be considered as, after literally absorbing one) from attacking or activating their special abilities. This needs a bit of set-up though, since it's a trap card.
That said, Kirby's got... A lot of powers, to say the least. I'm not sure if he could negate/null all of them.
