i3 and f1 also have encrypted disks. I have found some references on blogs for this, but the only place I've seen it mentioned by AWS directly is in this presentation from re:Invent 2017: https://youtu.be/o9_4uGvbvnk?t=30m20s (at 30:25 the presenter mentions that the nitro cards "protect the underlying flash device and customer data through encryption").