@kewms, @drmajorbob - let me take responsibility for perhaps not being clearer 
I was advised in another thread here not to try and integrate my current, live Scrivener projects into DEVONthink.
That helpfully contributed to my understanding of just how much ‘ring-fencing’ there is (and is not!) around a Scrivener project. (Although there do seem to be people who do so without any problems.)
It is indeed the outdated and no-longer-wanted Scrivener backups which interest me in this thread.
So - thanks to everyone’s contributions here - I shall feel safe deleting the defunct backups now