Yes, I think that's more likely than that the SC device is creating them; the latter would be more of a stretch. You seemed to disagree.
QM was specifically cited (Rutledge's quote) and the branch point is the start of the last 8 mins of Fentress' consciousness. SC presents a fairly mainstream interpretation (not universally agreed) of QM which throws up the kind of fanciful notions that make good sci-fi.
If the universes were already existing they would be markedly different (as 3cheeses alludes to above) since they would have emerged from earlier branch points, and it would be nigh impossible to identify what those points were. SC would have to somehow locate appropriate universes and open a window at the exact same time in those universes, at a later time each time in the primary reality - a massive computational stretch.