This is new for me too, so I’m just writing out my thoughts. When you’re referring to the pitch, do you mean the lead of the screw? But I would assume they are the same in this case. (What is the difference between pitch and lead when referring to a screw?). I don’t think that number actually matters for this reverse calculation (other than a sanity check)
I came to that number as well. But I’m not sure I’m following with the rest. So I’m writing it out to make sure we’re saying the same 
I assume the steps/mm are the microsteps and not full steps; correct?
the travel of one step is 0.0025mm as mentioned above.
1/8 microstepping thus means 0.02mm travelled per full step
with a standard stepper motor that does 1.8 degrees per step, we thus come to a lead on the screw of 4mm (the pitch could still be 2 if it has two starts, don’t know for sure if that’s the case).
I was browsing through the forum trying to find some pictures. Looking at this one: Linear Module defective and FW 1.8.0.0 Feedback - #6 by Edwin and trying to guess without a lot of reference (only reference I’m guessing about are the pcb’s that I assume have a more or les standard thickness of +/- 1.8mm)
Based on that picture, It does seem to have a lead larger than 2mm pitch. So a 4mm lead and 2mm pitch could actually be the case. (At this point I don’t have a compelling reason to open one of the modules to check )
So we seem to come to the same numbers independently, but I don’t understand that statement? Can it only do full steps for some reason? because:
I would assume it can make movements that small?
Probably there are hardware limitations I’m not aware off (I’m a software guy, not a mechanical/electronics engineer)
In conclusion, I guess I’m mostly rephrasing the same questions you have 
But based on that picture it does seem plausible that the lead screw has a lead/pitch ratio of 2.