Roger and Garp who were in their mid 30s (even if not prime, not far off), put their entire life savings into their final blow. Loki on the other hand? Washed HK Harald first with his DF only and after that obliterated him in base using Ragnir. Now imagine hybrid Loki going wild with Ragnir.
And yeah, I get it, Shanks "captured" Loki, but there is a reason Oda never showed it. It was just a Deus Ex Machina to have Loki rest until Nika's arrival, so they can go and kick the WG's ass together. At this point Loki being captured could mean anything and is probably not something Oda will further elaborate. We know how top tier encounters actually look like when they happen, they are nothing like what we would've hyped them up. Oda just needed to have Loki rest to be able to go all out now.
Lets face it. Loki is abnormal. Kaido became a meme.
The issue is you can't even see that you're stacking 25 different assumptions to reach absurd conclusions, that make no sense
narratively, let alone in terms of powerscaling.
Here are some counterarguments that you mostly just skip over:
1) Demon Xebec is stronger than mere HK Harald.
2) Like you mentioned, Roger and Garp were not in their peak.
3) Gaban and Shanks slowed down Harald's regen. This is not an opinion. It's Gaban's own words. They just needed 1 final shot.
4) Harald let Loki beat him. He literally told him "Hurry up, before I recover".
5) Loki doesn't even know what CoC is. Gaban made it clear. You must understand the power to be able to harness it properly. All his hype so far is 90% just DFs.
6) 10 years of pirating and he still has a Warlord level bounty (not saying he's Warlord level, just his bounty).
I can go on and on, but there are giant holes in the "Loki low diffs Roger" nonsense.