The key, explained in the blog post, is that every Bitmap that lives in ashmem needs to have an explicit .recycle call to unpin and free the memory. Fresco's DraweeViews automatically do this when they go off-screen.
Fresco limits the total amount of memory an app can allocate to bitmaps, even in ashmem. But although ashmem does count towards total PSS, it doesn't count towards the Dalvik heap limit, which is the bound that most Java apps have to deal with.