ACM has a wide range of conferences. There are conferences with an A* ranking, but also ones with a C ranking. Submissions to A*-conferences are not 80% garbage.
Your link shows that over 75% of the conferences are B or lower. So sure, the submissions to A* conferences are not 80% garbage, but these conferences are 4% of the total. Not really representative of academic work as a whole.
Oh hey, cool! My paper writing up my MSc thesis was at a B-grade conference with a 26% acceptance rate. That's actually more selective than I expected, given my opinion of my own work.
