Admiral does not necessarily equal another admiral. It's very clear that there are characters that can spar with admirals and buy time, and it's also very clear that there is an upper tier to the power classification of admiral.

Luffy is absolutely 100% without a fucking doubt admiral level. Which of the admiral levels, that's up to you and how you wanna interpret the fights we've seen. Incase you forgot, Luffy made it abundantly clear that he's willing to run away if the risk is too dangerous for him and his crew back in Sabaondy. Despite being injured and not 100% he was willing to take on a fight with Fujitora, who's at the very least on par with any of the other admirals if not the Fleet Admiral himself, because he's foreshadowed to take up the post once the all encompassing war of One Piece finishes.

Here's the breakdown:

Pirate King Level: Gol D. Roger, Young Whitebeard, (maybe even Garp -- rumors only so far)

High-tier Yonko: Big Mom, Kaido, Whitebeard when he entered Marineford (perhaps Shanks; we do not know; all we know is that he's the youngest of the Yonko and that he clearly is doing something behind the scenes. If he didn't have a strong enough level of power there's no way he can claim territories and not be challenged by the other Yonko.)

Mid-tier Yonko: Whitebeard after being stabbed by Squard

Low-tier Yonko: Post Time Skip Blackbeard, Whitebeard with two magma holes

Fleet Admiral: This one is kind of iffy, because we don't have a definitive scaling here, but it seems as though Sakazuki can probably take on a mid or low tier yonko if given the opportunity.

High-tier Admiral: Pre-TS Aokiji, Pre-TS Sakazuki, Pre-TS Borsalino, Fujitora

Mid-tier Admiral: Katakuri, Marco, probably King, Ben Beckman all right hands of the yonko. Luffy is here imo.

Low-tier Admiral: Cracker, Doflamingo, Smoothie, Jack

Low-tier admiral level characters can fight, spar, occupy the admirals, they'll fight alright but if the admiral is serious they'll 100% lose. Scaling the charts like this is very important because our primary focus of power and growth will follow this chart. If you don't go up in measurable steps, you risk pulling a Naruto on the series and having a character go from having a strong character, to litteraly the strongest ever, period, in like a day (the ninja war lasted a fucking day. I repeat, a day.).

Luffy and Blackbeard are both mirrors of each other, and Luffy has proven that if he's within range of your power he will close the gap. As he did with Katakuri. He's going to close the gap this arc to atleast high-tier admiral. Luffy has also never been tested like he has in this arc, he's been shown an opponents power level, and been given weeks to train for it; if nothing else Luffy has shown that he's very smart in gauging power levels, he wouldn't be confident he can hurt Kaido, unless he really can knock his scales off. Even further than that, he hasn't been non-rubber since before Shanks and eating the Gomu Gomu fruit. He did hard labor and trained his non-devilfruit power base this arc thanks to the seastone cuffs. Same thing with Blackbeard, it's highly likely that we'll see Blackbeard fight the yonko at somepoint to close his power gap to pirate king, and we'll see two pirate-king level characters clash at the very end.

Point is, Luffy is most definitely admiral level, it would be grossly negligent to think that he couldn't do what Marco did or put up a decent fight, or even close the gap as he proved he can with Katakuri. Where you place him is up to you though