It really doesn't matter, regardless of how much you press the point. The matter is irrelevant.
That doesn't mean I shut it down - the thread is open and anyone is welcome to respond (and so am I!) You're free to express your interest in the technical minutiae involved in a small part of making this work, and I'm free to express the viewpoint that the answer has no utility.
There's a reason why in all the interviews and books about this body of work you won't be able to find whether or not they used a cyan filter, and weather or not on some shots there was a bleak fraction of sunlight filtering through the cloudbase.
Sometimes, the real answer to the question is in the lack of a clear answer. Again, you're free to ask it anyway, and anyone is free to go into the depths of the spectral sensitivity of the film used etc. I still think it's a silly exercise: regardless of how they did it, what they did was deliberate as they made that explicit. As for the technicalities, we all know at least a couple of ways it could be achieved - so from that angle, there's also very utility (and at worst, a distraction) in the exercise of figuring it out.
As to your question why I don't simply ignore it: you're right to ask. For me, it's perplexing how there's apparently a tendency to look for some kind of technical deus ex machina in work that's so evidently not about technique. Moreover, they've been explicit as to this aspect of the "how", which happens to boil down to something entirely unrelated to gear, so the attempt to dig until we hit upon camera hardware (or other bits of photographic paraphernalia) is downright puzzling.
Even more so, insofar as technique played a role, it was to render it as transparent as physically possible - essentially to remove it from the equation. The makers of these works did this deliberately. In interviews, the Bechers never directed the course of the conversation into the direction of technique - which surely they would have done if they had believed it to be essential. They answered direct questions about it, but for the most part appear to have discussed the heart of the matter, i.e. (my interpretation) an ontological investigation of our constructed environment. How does a filter enter that story? Does the narrative surrounding this work somehow benefit from the observation that somewhere down the line, it involves a cyan filter to blank out the sky? I don't think so.
The reason I pick up the subject (which is not about you personally in any way), is that I recognize this as a systematic characteristic of the discourse of artistic photography among esp. amateur photographers. That I think is interesting. Not the question of the filter or the development or whatever - but the fact that apparently, for some reason, the question is inescapable on a forum like this one.