Sorry, I don't know what you mean. When I said it provides marginal privacy improvements, I meant the caching, not the disklessness.
Diskless does provide privacy improvements, as it drastically reduces the odds of something accidentally persisting to storage.
Diskless (edit: with OS in initramfs) is indeed a golden standard against local persistence, but requires quite a bit of extra RAM - few GB for "latest Ubuntu LTS".
With regards to preventing accidental persistence, disk with only dm-verity partitions is as good, with extra advantage of only adding a little bit of extra RAM usage (/tmp, /var/run, ...)
For that matter, even something as sloppy as booting with rootfs wich can't be remounted rw (iso9660, squashfs, etc..) and is the only mounted fs, is also perfectly good against accidental persistence.
You could run from NFS and not need much extra ram. Plus you save like $25/node by not having a local disk.
You could go the extreme and boot off Google Drive (or any other fuse FS).