Even if you disagree that Marco is well and truly admiral level, the level in which people seemingly need to be in order to be offered the position clearly varies quite wildly.
Lets look at Fujitora, Momosagi and Chaton as examples.
Momosagi and Chaton were admiral candidates, and people might say that it implies them to be very strong, but then they also lost those positions to Fujitora, who most people call the weakest admiral anyway. Thus, they were weaker than the weakest admiral.
Lets not forget, that they also lost this position specifically to a Fujitora which didn't even have a devilfruit. Feasibly, they were weaker than the weakest admiral, when he didn't even have his devilfruit.
Finally, we have to notice the side benefit that would've gone into picking Momosagi and Chaton: namingly they are provenly loyal, having already been renowned Marines. Yet a fruitless Fujitora, who wasn't even part of the Marines yet, was still picked over them.
That gives off the impression that Momosagi and Chaton are MUCH weaker than Fujitora actually is, or any of the admirals we've seen, while we're talking about it. They definitely weren't above Yonko Commander Level, yet they were still offered an admiral position, even if they ultimately weren't chosen.
If Marco was in the Navy, theres a good chance he wouldn't hold anywhere near the same amount of experience, or have ever found the ultra-rare Pheonix fruit, but assuming he was still just as strong as before, he would definitely be offered the position of admiral.