As an obvious example, I uploaded a photograph I took of a painting I have by Julie Hagen-Schwarz* that's been in private possession since 1882 and never put online: https://www.stableattribution.com/?image=7730efac-bf52-4077-...

It still finds ostensible "source images" for the art. So, yes, it's clear this service is pretty much bogus.

* https://en.wikipedia.org/wiki/Julie_Wilhelmine_Hagen-Schwarz