The rest of the paper is devoted to primordial black holes, of which none have been seen and I think none now is expected,
It is impossible to know even the presence, much less the origin, of any black hole without some form of known radiation to "find" them. This necessarily means that any BH, regardless of source, would be impossible to simply be "seen".
Indeed, the only way BHs are known to exist is their impact on objects we CAN observe by some telescopic means. Simply put, primordial black holes
can never be ruled out, at least not by rational science. No empirical evidence has yet, and probably ever will eliminate their likely presence from the known universe.
The "father" of BHs, none other than Stephen Hawking himself, predicted that their smallest mass would be around 10E-8 Kg (1). To be sure, Hawking was not convinced BHs even existed in his early career, only to become their greatest adherent. It seems likely the same events will play out with PBHs. Denials followed by grudging acceptance.
This acceptance might begin with more data from BH mergers, demonstrating larger and larger BHs merging in the distant past. Once these are found, it will become increasingly more difficult for the real experts to deny the probable existence of PBHs, especially those at the largest scale.
1.
https://en.wikipedia.org/wiki/Primordial_black_hole