I actually started with 5e, but I believe Van Richten's Guide to Ravenloft does note that the Dark Powers don't cause people to fall, they tempt them. I've only read excerpts though and have yet to buy it.

That said, the wizards created the temple to seal the vestiges. If the vestiges are the Dark Powers, how could they be born in a place that didn't exist until they were already sealed away? By the established chronology, the vestiges had to exist before the Amber Temple, and therefore cannot be the DPs as they were born in the temple. Furthermore, there's a line in the text of the module that notes when Strahd entered that "The evil powers in the temple felt something much stronger: a darkness that eclipsed their own." The stalker sociopath warlord is that evil? Really? I'd argue this was the moment the Dark Powers were born, but they are separate entities as supported by the text.