Thank your for your answer. I must say I'm almost shocked that this idea had been brought up and discussed already as a serious hypothesis. I can only assume that I some how picked this up when watching one of the many documentaries about black holes without actually remembering. Very interesting.

Right after sending my initial comment, I thought about the following. I assume that after the big bang, the universe first had to cool down below a certain threshold so that black holes could even start to form. Therefore, a precondition for the primordial black hole theory would be that the inhomogeneities can not have started to materialize before the temperature hat fallen below that temperature. So my next question would therefore be, do our computational models produce enough temporal resolution to be sure regarding the order of these events? Or does this thesis still require to simply assume that black hole formation preceded the inhomogeneities?

EDIT: I just skimmed the wikipedia article which you had linked in your answer. Thanks for that! So we are talking about events within the first second of the universe's existence. That's beyond mind-blowing. 😅