Well, maybe and I'm not saying you're wrong, but...
Not that RealOEM is the final word on this subject (I have been wrong before when I quote it), but according to them the front springs between a Euro E28 535i (manual transmission 7/85) and M535i (manual transmission 7/85) are the same: Part #:31331126904.
Whereas for the Euro M5 has listed: 31332225115, 31332225024, and 31332225233 (assuming all are M-Tech and are listed as applying to only the M5).
The same comparison applies to the front stabilizer bars: 19mm for M535i's and 535i's versus 21mm for M5's.
My understanding of the M535i model (as a "standard" list of options)is that the only differences between stock 535i's were:
-M-Tech Body Kit
-M-Tech Wheels
-M-Tech Interior (which is only the seat badges, since the front seats are standard BMW Sport Seats while the rears are the same for all E28's)
-Close-ratio 265/5 or 6 Transmission (not always, since you could order an M535i as an automatic; many were)
M-Tech suspension wasn't always part of the package. Also any of the above items, including the suspension, could be optioned on any E28 535i so it is hard to apply any specific, definitive M535i "package" as unique to the model.
I would think that some dealers, sellers, etc. might have ordered M535i's with the M-Tech suspension to help "sell" the model and set it apart from stock E28 535i's. My understanding of how Euro dealerships worked back in the 1980's is next to nothing, but perhaps dealerships created a list of options to be included as part of the M535i that the salesmen would walk through with the customer; maybe the suspension systems ended up on this list more often than not
But from BMW, M-Tech suspension wasn't (necessarily) an official part of the M535i.