It's a settlement deal offered by the WADA, who saw their case shot down by their own director acknowledging that there will be more and more nothingburger issues with extremely sensitive tests. Really the WADA shouldn't have appealed in the first place, but this I suppose is an acceptable way out for them. And with CAS being very unpredictable in this case, I'm not surprised Sinner took the offer. He's been cleared of cheating and PED abuse, he acknowledges his responsibility for his team's errors, takes the loss and moves on.
Sinner will not miss a grand slam. His ban runs from February 9 to May 4, and ends in time for him to return at his home Masters 1000 tournament in Rome, then play at the next major tournament at Roland-Garros three weeks later.