Oh of course I know DH are not liable!
But I'm inferring from this sentence -- "but they aren't backed up, and hence aren't restored from backup if we have to do so" -- that what happened is that shit happened: they had to backup or move all the stuff on some storage resource or other for some reason, and on the subsequent restore of that backup, my /tmp/ stuff didn't make it back to Earth.
Of course it would have been cool if this 'feature' was documented somewhere, but if I have to be super honest, I probably would not have read that documentation and would have been none the wiser..
Well, may this thread serve as a warning to anyone weird enough to store quasi important stuff in a directory named "tmp", and as a reminder to make backups (although, it seems the DH-one-button backups that I have stashed -- somewhere -- may also have omitted my poor files)
I think I'll do my future backupping in a more manual and hands on way and apart from this I guess shit happens and that's life