Basically, the fabric of the strap constantly rubs up against the case back, and when dirt gets trapped in between, over time, that causes scratches and even marring on the case back. According to a dedicated article by British GQ, this kind of marring cannot be prevented by simply cleaning the strap. They say no matter what, the fabric will cause wear to the case back. Are they right?
I only have one NATO strap that I wear infrequently, but yeah, that’s completely plausible. Now, what the extent of that will be, I dunno, but on steel watches that I wear a lot (on straps), the caseback is covered with tiny swirls and scratches just from setting down the watch on a wooden surface for years.
At the end of the day, I wouldn’t worry about it.