Well, I'm not saying across-the-board that 'snails scratch acrylic'. My snails haven't scratched mine, AFAIK.
What I meant to say is that a snail that has sand caught near its mouth/radula/shell (as in the picture) could very likely scratch acrylic.
I'd be less worried if the sand was under its foot, considering that the foot doesn't slide across the surface the way the mouth/shell parts do.
The bottom line, in my scratchy acryclic book, is that it doesn't really take any downward pressure to scratch acrylic. Simple contact with a hard material is enough.
That said, these scratches are likely to be extremely shallow and hard to see, except under certain lighting conditions.