I'm not sure if even Casimero believes this. Fulton held two world titles when Inoue beat him as did Marlon Tapales. I don't see how Casimero can be above them when he holds zero world championships and didn't look great in his previous fight against an unranked opponent. At this stage in his career the only motivation for still chasing an Inoue fight is the possibility of a seven figure payday. If we're being realistic, his chances of beating Inoue are really slim.
Here are the list of candidates:
1. Terrence Crawford - beat Spence, another pound for pound boxer, and then captured and unified all the belts at 147 lbs?
2. Devin Haney - undisputed at 130 lbs beat Loma, move up in weight and then dominated Prograis to win the WBC belt?
3. Naoya Inoue - move up in weight, challenge the best champion at 122 lbs in Stephen Fulton, then unified the belt against Marlon Tapales, second man to do so, unifying belts in 4 belt era in 2 different weight classes, second only to Crawford.
Devin Haney is a distant 3rd for me. His win against Loma wasn't convincing. It's a tossup between Inoue and Crawford. Inoue was more active and won all 4 belts in a year. Although I consider Spence a manufactured champion who was protected from Crawford for many years, due to PBC's influence with the sanctioning bodies, he was still highly regarded. Crawford beating him so brutally gives him a slight edge over Inoue in my opinion.