I don’t really see what the problem is here. What is being argued is that the “irreducibility” of some extant system does not constitute a barrier to it’s evolution.
The argument is that if we can show that systems composed of multiple interacting parts that contribute to it’s overall function, which will stop functioning if certain parts are removed(so they are irreducible), can still evolve, then we have shown that at least this aspect of the argument from irreducible complexity is not a challenge to evolution.
So it does not suffice to merely point out that ATP synthase(for example) has a function which, if you remove parts of it, it stops being able to perform. Because it has already been shown that this kind of mutual dependence relationship between individual components can evolve, by cooption(exaptation) of components that themselves performed other functions (and which they still some times do), or used to be part of other systems.
This is why Behe went on to move the goalposts and insist that there must be some sort of degree of complexity(number of parts of the system becomes the new crucial aspect) beyond which irreducibly complex systems cannot evolve.
Behe originally argued his point about the irreducibility being unevolvable by focusing his argument on a Darwinian emergence of the system, whereby he argued against a gradual increase in the basic function as the system elaborated. He said the system’s irreducibility prevented this type of gradual “Darwinian”(cumulative selection-dominated) emergence where the function came early with a few components, and then gradually improved (with each step being selected for cumulatively because the system functioned better and better) as more and more components eere added. His point of course being that if you remove a component and the system stops working, then there is no such step-wise Darwinian pathway to the extant complex system’s function.
Biologists have responded in two ways. First by pointing out there are neutral pathways to complexity, and by pointing out that parts can be coopted from other functions, and the system as a whole can be exapted to change function along the way.
Behe has responded to the point about neutral evolution by saying neutral evolution just makes it very unlikely because you’re missing selection to aid the fixation of each new step. And Behe has responded to the point about exaptation by arguing that (paraphrasing) the probability drops off with each added component. (A point which I have above tried to argue seems without basis).
Biologists have responded to Behe’s claims about neutral processes in evolution by saying Behe ignores constructive neutral evolution, which can be inherently biased towards complexity. I have not seen him respond to this.