Marco's regeneration isn't passive, it's active.
It's entirely possible that Marco didn't want to waste energy healing a tiny non fatal injury like what Garp inflicted on him and instead decided to let it heal naturally, we've already seen in a recent chapter that Marco stops regenerating when he's out of stamina, so why would he waste said stamina healing a superficial wound?
Garp cracked his neck.
I highly doubt a punch from garp is less fatal then a shot in his shoulder.
As I said, there are other explanations other than Haki negating regen, hence the possibly at best.
Outside of "he didn't want to waste it on the strongest marine's attack", then there's no other reason.
Also, why are you assuming Kizaru wasn't using Haki on his lasers?
Cause we don't know if he can do that.
People don't assume people (which can use haki) can use Haki when they hit Luffy and bypass his rubber even though everything points towards it.
Issho can't put Haki on his gravity, which is why he didn't hurt Sabo with it.
Whitebeard didn't put Haki on his quake when he hit Akainu, which is why he shattered.
Assuming everybody can infuse haki into everything they do is just... headcanon.
Also, lasers aren't solid. We only see Haki on solid objects.
We know Haki can be applied to projectiles as shown by any notable swordsman and the Kuja Pirates, not just that, but devil fruit projectiles as shown by Katakuri.
Notable swordsman which require ridiculous amounts of training for it.
Kuja pirates who put them on regular projectiles.
A Special Paramecia who's creations are initially connected to him and he can infuse haki into it.
Gravity can't have haki on it.
Quakes can't have haki on em.
Also, again, lasers aren't solid.
If your argument really hinges on the assumption that Kizaru wasn't using Haki to attack Marco, well, this possibly for regeneration negation will last about as long as it takes until we next see Kizaru fight, because it's extremely obvious that now that Haki's a big thing and is visible unlike Pre-Timeskip, we're going to see the Admirals use it on their element, they'd be completely redundant if they weren't able to.
I wish this was the case
A while ago we had a CRT go in place where the admirals scaling 1/3rd of WB off of their Haki usage was declined because we don't know if they use haki for their punches and kicks.
Kizaru kicking Luffy isn't accepted as Haki usage even though that's a main reason we assume so, so that's weird.
Kizaru hasn't shown to put Haki on his lasers. And until he does, we don't assume so.
