The ninja bros downscale because they hurt a suppressed FF, Bang downscales because he hurt a weaker Garou.
Suppressing himself wouldn't lower his durability. You can hold back your AP and you could hold back your speed, but if you don't have any kind of special power, technique, or something notable that interacts with your durability, you can't hold that back. At least, FF hasn't shown to be able to do that, and idr if anyone in OPM has.
The ninjas are just stronger than everyone assumed. And Flashy Flash himself did note that if he didn't kill the ninjas, the other S class heroes would have serious trouble, so it's consistent. We never Iirc, they were both Cadre Candidates too, so we can't even say they should have 'rank inferiority' or anything like that. In Chapter 96 (94 in Viz), they state that against both his speed and
power that the other monsters (as a blanket term that includes the cadre) would be helpless against him. And this was still a suppressed Flashy. It might've just sounded like them being cocky before, but considering that Flash really does scale to Platinum S and Garou on the lower end at full power, it's not farfetched to say that the level of power Flash was showing to them could've very well been above the other monsters (that they knew of). Even if you wanna say it was just them being cocky, Flash did say that the other S class would have a hard time dealing with their speed. But it's just extra supporting evidence or interpretations.
Flash when he stopped holding back used Flashy Kick Barrage on them, and it stunned or damaged them but didn't make their bodies completely explode or get blown apart. This could be compared to the Flashy Fist he used on Platinum. If they were leagues apart ap to durability wise, Flash when he got serious wouldn't have needed to use the kicks as a set up move and would've just overwhelmed their durability with his raw AP like other hand to hand fighters do to monsters. And then Flash one shotting the Ninja's with Flashy Slash just confirms what Platinum said about Flash's AP being far higher with his sword than without in the fact that he can one shot people that could take multiple of his bare handed strikes.
Also on a different note, should Flash and the ninja's get limited explosion manipulation (via kinetic energy) for what they did in the cave? Flash also did it again with the Flashy Fist attack, and the way it's drawn is different from other characters that destroy the environment from their strength (besides Garou's stomp at the beginning of the fight, but he was also moving at those 'kinetic explosion' speeds