That's what I am referring to. Instead of looking into some idealistic "political definitions" and say "according to this this system isn't capitalism", we should look into what's going on (in reality) and say "this is what capitalism looks like".

Else it's like those marxists, that instead of accepting that communism can result in a BS bureaucratic state, they said that USSR and co weren't "real communism/socialism", because they didn't match the theory, and that real communism hasn't been tried etc.

That's what I call going backwards. One has a definition, and compares reality against it.

Instead they should derive their definition from what happens in reality -- and maybe even accept that their idealistic definition can't ever happen anyway.

It's like some zoologist had a definition of animal X, and then matched it with various animals and said "this can't be X, because it doesn't match the definition". Well, if that X existed only in theory that's a bad practice. The definition of any X should come from studying actual animals (e.g. this animal species has so and so characteristics, we'll call them an X, e.g. tiger). So, where has this ideal "capitalism" even been seen in the wild?