Ever since back on Orojackson, when people would call me crazy for suggesting Marco was the "Rayleigh" of the Whitebeard Pirates, I've always maintained he was the strongest of the Yonko First Mates. It always made sense for a multitude of reasons. During Marineford, he is the only character that Oda nerfed on the spot. Plenty of other strong characters on Whitebeard's side were dealt with, or held off by traditional means, but Marco was explicitly slapped in Seastone cuffs in order to be taken out of the fight. This now proves it wasn't done for unjust reasons. He had great showings against Admirals, and now a great showing against a Yonko.
After Marineford, he was the only character explicitly named by the Gorosei when speaking about the WB remnants, and taking down Blackbeard. His name was put alongside the other Yonko's in that regard. These guys have been around much longer than Marco, and have witnessed his exploits as the Whitebeard Pirates rose to dominate status.
Speaking of the Whitebeard Pirates dominate status, the strength of Whitebeard's crew played an integral part in his standing in the World. Sure the main player has always been Whitebeard, as he was just a cut above everyone else, but even after old age and sickness caught up to Whitebeard, he still had the freedom to hang out in the middle of the Ocean, and paint himself a clear target for the rest of the World, and yet nobody could/would come take that from him. Having another Top Tier (Albeit low) in his crew is a perfect explanation for this. The other Yonkos and the Navy aren't going to come for Whitebeard's head if he has another Top Tier in his crew.
Now Oda has brought Marco into this war on Wano, to specifically give the Alliance a chance. On Kaido's side you have Kaido, Big Mom, King, Queen, Jack, and Smoothie, who are all high caliber characters, and who does Oda bring in for the alliance? Marco... He gives them one High tier, amidst two High end Top Tiers, and 4 High tiers? That doesn't help at all. It only helps if Marco can actually make somewhat of a difference, and this last chapter proves he can.
That being said, I don't think Marco is capable of defeating any of the Yonkos or Admirals in a one on one fight, but I think he stands a far better chance of staving one off, than any other character in the series. This is just the kind of portrayal Oda has given him.