There's a reason Marco was the right-hand man to the World's Strongest Man.
1. Let's start with the obvious. Marco's AP. Easily Marco's biggest weakness...or is it? Well, for one, Marco had enough power to push off Big Mom's ship, which is MANY times bigger than the Thousand Sunny.
2. Furthermore, just like King, Marco's strength gets enhanced as it's a Zoan Devil Fruit. But these two points alone mean little to nothing if King has his flames on. So, let's address that.
3. Something a lot of people forget is that Marco actually has shockwave attacks, which we've seen in One Piece do internal damage, as seen with Sengoku against the Blackbeard Pirates.
Despite Queen's Dinosaur durability, Marco was able to bypass it using his shockwave attack "Phoenix Brand," which he has only used on Big Mom's ship and Queen in this one instance, so far. Thus, as shown before, Marco would be able to do internal bleeding to King with his flames on.
4. Now, what about when King's flames are off, and he gets faster, what can Marco do about that?...Well, it's pretty simple. Marco has shown greater speed feats than King; he was able to speed blitz Kizaru's lightspeed attack after all. King has shown no speed feat close to this.
5. So, Marco is able to deal internal damage to King with his flames on, and if King turns them off, Marco is likely still faster than him. To add to this, Marco is a terrible matchup for King because King is a flame user. And Marco has shown his phoenix flames counter flames.
6. Now at this point, some of you might be thinking, "But hang on, didn't King already beat Marco?" To which I would say no, he didn't. After Marco fought Big Mom, King, and Queen together, King and Queen together again...then held Queen back, we cut to this. What people like to leave out, however, is the context of this with the text next to Marco, indicating he was never beaten; he just reached his limit. Furthermore, there isn't even any implication he and King fought. The last we saw King before this (chapter 1022) was chapter 1015, 9 chapters after Marco 1v2'd him. In fact, in the same chapter where we see King, we also see Marco chilling around Sanji, helping the alliance and using his flames. Then after this shot below, the next we see Marco is when he ran out of stamina. So there's no indication they ever even fought.
And King didn't even go after Marco after Marco fought him and Queen together. So the timeline is: Chapter 1006 Marco 1v2s. 1007 Marco holds Queen back. 1015 Marco is with Sanji and fodder, while King is elsewhere. 1022 Marco is out of energy. They never fought 1v1.
7. That should've been obvious as Oda said Marco ran out of stamina and not defeated by King. But even then, what people fail to point out is just THREE Pages later, Marco, despite out of stamina, gets back up and speed blitz and blocks King's attack.
8. Furthermore, seriously injuring and killing Marco is no easy feat. Big Mom herself let him go because it would take up most of her weapons.
Kizaru required assistance from a vice admiral to put seastone on Marco just to damage him... an admiral needed assistance because Marco is too OP. (Obviously, Marco isn't admiral level, just like to point that out, and this isn't a diss on Kizaru, the same would happen with anyone)
9. Let's not forget that the Gorosei name-dropped Marco with the WB Remnants as a possibility to defeat Blackbeard. Obviously, they failed, but it was offscreen Blackbeard after all.
10. Furthermore, Marco actually has insane feats. He fought against 3 OG admirals, 1 Yonko, tanked attacks from Old Garp and Kaido, and fought a YC1 and YC2 at the same time. Did he do damage? To Queen, yes, but everyone else on this list has top-tier durability, and hax as well.
And Marco does too. Sure, Marco didn't damage a lot of them, but Marco has also come out of a war where he fought 3 admirals, and a war where he fought a Yonko and tanked a Yonko's hit with no permanent injuries because he's overpowered.
11. And let's not forget. While King and Queen got no-diffed by an admiral 2v1 a week with recovery, he even has bandages, Marco was surviving against 3, 1v1. While yes, King doesn't have his sword, he still has his DF which he used to attack multiple times, so does Queen.
12. Even further than this, Marco is the sole protector of Sphinx Island, the Marines knew where the island was but because Marco is so OP they refused to raid it until they knew Marco left...that's semi-Yonko status they've given him.
In fact, they sent Greenbull after Weevil, but this whole time didn't send him after Marco, because the marines just like Big Mom know how annoying it is to deal with Marco.
14. And my final point, which is a little meta, is that Marco still has narrative relevance unlike King, with the Stussy and Weevil plotline. Oda hasn't shown his arsenal yet, nor given him a fair 1v1.
15. To conclude, Marco has attacks that can bypass King's defense. Marco has greater speed than King, he's shown greater recovery, He counters King's flame attacks, he still has narrative relevance, and overall, he has better feats.