That's like saying Smoker earned himself as current Luffy level because he could beat him preskip. Garp's 38years old ago portrayal(which was faked out by world government to boot) meant nothing compared to prime Whitebeard/Roger. In his prime, Roger was viewing Garp nothing different from Sengoku when he was thinking he could die in a confrontation with Whitebeard. Even old and sick Whitebeard was considering stronger than Garp by Garp's long time partner(Sengoku)
Their portrayal as one of the two factions you need to surpass to become pirate king says otherwise
Marco's portrayal as the final rampart in Teach becoming the pirate king is superior to any other YC1s, his portrayal as Whitebeard's right hand, when Vista can match one of Shanks' top3 prove otherwise, his match-up with Weevil, who you rank as low top tier will prove otherwise
Garp’s portrayal as the strongest Marine places him on at least Yonko level. The rest is debatable I guess.
Oda has consistently given the Yonko more hype/portrayal than the Admirals:
1. Sengoku stated that they could still get annihilated even though they had all of the Admirals there.
2. Garp didn’t think that the Marines could handle 2 legends at the same time. Even two years after MF, the Marines, led by absolute justice extremist Akainu, still haven’t bothered to try capturing the PK’s right hand man even though they know where he is.
3. No Admiral has held a “World’s Strongest” title
4. When Aokiji snuck up on Doflamingo, placed his hand on his back, and told Doffy to stop attacking Smoker, Doffy instantly disobeyed him and attacked Smoker anyways. Immediately after Doffy broke out of Ice Time, he was smiling like crazy. Compare this to Doffy’s reaction to the idea of him merely disappointing Kaido. Doffy also has no qualms with trying to kick Fujitora, and then had plans to take our Fuji with help from his crew. Doffy has shown way more fear for Kaido than he has for Admirals
5. The Gorosei tell their own Fleet Admiral that his face means less then nothing to them, but they value the opinions of one of their greatest living enemies.
6. Base Kaido casually one shots Boundman, which was YC1+, and O-Lin defeats Beast Form Queen with two half-assed attacks. 20 years ago Base Kaido also one shot Oden with a sneak attack, while Marco, someone noticeably below Oden, is smiling after tanking Kizaru’s strongest shown attack, flies through another attack, takes multiple more attacks offscreen, and then only gets put down by a sneak attack because Onigumo puts seastone cuffs on Marco.
7. It takes Fuji and Ryo a while to put down Sabo and 3 Vets
Luffy is being called 5th Emperor despite only defeating 2 BMP YCs in tough fights instead easily beating the entire crew.