Forget about Luffy, he is the MC, and he will lap the old legends. Welcome to shonen.
In regards to how Loki compares to the other top tiers from the past. There is just way too much evidence at this point, one thing we can overlook, two things we can say is a coincidence, three, four, five, we're like 6+ "coincidences" deep now. At this rate Oda can come out and say people are straight up wrong and you'd still think he means something else.
Oda has conditioned this fandom so badly that he can put all this effort into conveying something, and they still don't believe him, which just speaks to how much he has traumatized this fandom 😂
However this isn't ZKK, Oda is literally doing everything in his power to tell his audience: Loki is that guy. He’s being positioned above the old legends, and these aren’t hunches anymore. The flashback is spelling it out in multiple ways.
1.There is a reason Oda made him replicate Kaidou's intro feat but as a newborn.
2.There is a reason Oda has an old legend himself, a top tier who fought with Roger his whole life, have a full time role wanking Loki.
3.There is a reason Oda spent the entire flashback hyping up this legendary fruit as the key to taking over the world, where someone like Harald wasn't even confident he could attain it without amps.
4.There is a reason both Harald and Xebec were amped, their downfalls paralleled. Them taking on multiple top tiers and still having the upper hand.
5.There is a reason Oda has Gaban + Shanks playing the role of chicken wings so Loki can come in and save the day.
6.There is a reason Imu had no issues with constantly sending even fodder Holy Knights up against Roger and his crew, but he doesn't dare do so around Loki.
7.There is a reason the final villain who has been alive for centuries is twerking to Loki when he was indifferent to every other legendary pirate.
Don't overcomplicate it. If you isolate any of these reasons by themselves sure you can sit here and find a reason to excuse it. That's not the point. You combine all of these things and the picture is clear. Oda is not doing all that just to say Loki is peers with Big Mom 😂 , especially given that current Loki is even older than those events.
The Shanks capture comment isn't enough to outweigh all of the above. It's suspicious in the first place why no one has ever said Shanks defeated Loki, when people have no issues saying Luffy defeated Kaidou, and even that didn't actually mean Luffy was stronger than Kaidou. Worst case scenario, Oda is about to make Shanks above everyone as well (this is the more unlikely path but I'm not about to act like this is my first shonen and that this isn't possible either).
This is just another generic shonen, yes power lvls get wonky near the end. Y'all need to stop holding it to some special standard, those legends are not the benchmark...that much has been cleared up, Oda has went out of his way now in this flashback to show it. The minute Luffy defeated Kaidou, someone who was already around the same lvl as the strongest of the old legends, before the final saga even began, this was bound to happen at some point. Oda couldn't even do Roger the courtesy of separating him from like 5 different characters in this flashback alone. He has spoken.