Sapphire crystals get scratched and you don't need something as hard as diamond to do it. If you take one of the Mohs picks that are on the lower #s and apply enough pressure you can scratch sapphire. Similarly, out in the real world if you hit the crystal with enough force it can lead to scratching. I've used the first generation steel AW, S2 White Ceramic, and have the S3 gray ceramic. The first 2 got scratched. If you look up fer557 on youtube, I did a video of the scratches on my S2 white ceramic. I, however, am much harder on my watches that I think most people are. My mom has a first generation steel, she's been wearing daily for 2 years. After one year, apple replaced it since it HR sensor fell off. I don't even see hairline scratches on her crystal and I don't remember her scratching the previous one. And she wears it for everything, working out, gardening, etc.
The sapphire apple uses, I think is pretty good, perhaps not as good as other sapphire watches I have used, but the difference is very small. Not all sapphire is created equal that is for sure. The other issue is most watches have a bezel, some protrude more than others, but they help protect the crystal. The AW crystal bends into the case so at the corners there is no bezel protecting it. So it makes sense that that is where I typically get deeper scratches since that area takes a more direct hit.
Finally, in regular use I don't see these scratches, I have to get a light and examine the crystal at an angle. I'm not bothered by the scratches, it's part of having a watch. The white ceramic case of the S2 on the other hand, is the most scratch resistant case I have ever used.