Diskless does not mean SSH-less or network-less. The "data" can be pulled / pushed just the same, which is to say, Diskless, in this case, is no better than verifiably read-only partitions (like on ChromeOS & Android, for example).
Diskless does provide privacy improvements, as it drastically reduces the odds of something accidentally persisting to storage.
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.