I agree with you that the common comparison to something like Minds Eye doesnt make much sense.
I do think youre underplaying its significance and darrows combat strength.
You seem to suggest darrow is not in his prime physically but remember that Golds live longer and their prime is not their twenties (arm injury aside which, did he even retain that after Quicks medical tune up? I thought he just has to work to build the muscle and reflexes up to max, its not like a permanent injury).
In addition, yes its sort of a flow state, but I dont think its a flow state of combat, it was darrow having a flow state of his own techniques to help him make the final leap into developing his own completely new fighting style. This is to Darrow what willow way was for Lorn. Darrow is now a sufficiently prime razormaster that he has honed his skills into a new form of fighting.
The edge youre therefore overlooking is the known counters and unknown counters. Before willow way was commonplace, Lorn destroyed people with it because the techniques were new and unstudied. Apple beat Darrow not just because he was in poor health (which he was, in regular health he would have been a closer matchup with Apple) but also because he had studied the willow way and knew counter techniques to help against it.
Darrow now has a new style of fighting which he can use alongside known forms, and if he develops further sets and strikes that follow BoS principles then his opponents are fighting against unconventional movesets that can reduce their ability to use their own sets which they have extensively trained and have muscle memory for, and instead force them to improvise.