The Dark Powers have never been all that clearly defined.
There is no official documentation nor official statements I'm aware of that describe a whole lot about the nature and history of the Dark Powers. That makes this question difficult (if not impossible) to answer definitively.
I have a searchable copy of Domains of Dread from D&D's second edition which contains this section in Chapter 1 (the searchable copy seems to have some formatting differences from a print copy, so I can't give a page number citation). I'm not aware of any clearer or more definitive information from any later edition, so this is the best support I could find even for a question tagged for 5e:
Defining the Dark Powers
Is it possible, therefore, to learn the secrets of the dark powers through a careful study of the macabre place they have
created? One would think so, but proving the validity of the conclusions produced by such a study would be difficult or even
impossible.
The precise nature of the dark powers is debated by sages and scholars throughout the land. Sages have argued about it since
the earliest days of Ravenloft's existence. Quite simply, no one seems to know the answer to this question. More
importantly, it is impossible to say whether any given answer is right or wrong. The Demiplane's most educated residents
have uncovered conflicting evidence, supporting any number of possible answers. Consider the following arguments:
The Dark Powers are Evil: Certainly, evidence exists to support this belief. After all, the dark powers have gone to a great
deal of trouble to create a vast region in which to concentrate the evil of the multiverse. Could this be something of a
recruiting center from which an incredible army of malevolence will one day be released? That possibility could be the most
frightening conclusion one can reach.
The Dark Powers are Good: An equal number of scholars put forth the assumption that the dark powers are actually
creatures of good, not evil. After all, they have imprisoned some of the most terrible beings in the multiverse. In fact, not
only are these creatures imprisoned, they are endlessly tormented. Can any force that so punishes evildoers be considered
anything but good?
The Dark Powers are Few: Some scholars believe it is incorrect to use the plural when referring to the masters of Ravenloft.
It is their assertion that a single entity, whether good or evil, rules the entire Demiplane.
This theory is especially popular among those who believe that Ravenloft was created for some special purpose. If this is the
case, it is easier for them to believe that a single entity is working behind the scenes to coordinate these events.
The Dark Powers are Many: Equally or even more popular, however, is the belief that the dark powers are numerous.
According to this theory, no single being could accomplish all that the dark powers have.
Of course, exactly how numerous the dark powers are remains a subject of much debate. Some say that there are three, the
number in a coven of hags, while others insist they are a legion. To date, however, no one has provided any evidence to
support these claims.
The Dark Powers are Imaginary: At the root of all debates about the dark powers, of course, is the question of whether or
not they even exist. Many say that they are nothing more than a manifestation of the darkest side of the imagination.
To be sure, no physical proof of their existence has surfaced. While many people claim to have seen the dark powers, none
of these reports are verifiable or even especially reliable.
The Dark Powers are Real: Of course, if the dark powers do not exist, it is difficult to explain the creation of the Demiplane
of Dread. Most Ravenloft scholars agree that the dark powers, whoever or whatever they are, do exist.
The two statements listed in the question aren't hard to reconcile when we don't have many constraints on what the Dark Powers are, but the "correct" way to do so should be chosen to fit your campaign rather than to match a canonical explanation that doesn't exist. A few possibilities that sprang to my mind:
- The Dark Powers could easily be a group that has accepted new members
over time, with a specific subset being relevant to Curse of Strahd
but not describing the Dark Powers as a whole.
- They could be fundamentally a part of, or derived from, the
Shadowfell, but have a specific manifestation on the Material Plane
which coalesced in the Amber Temple at some point (regardless of
anything else that may have been, or later come to be, trapped
there).
- The evil vestiges may or may not have been some or all of the Dark
Powers, either originally or through some later development.
- The Dark Powers may have a distinct manifestation among the Domains
of Dread, and so have been "born" in the Amber Temple after Barovia
was withdrawn from its original plane of existence.
- And, of course, what Exethanter "knows" about the Dark Powers and the
Amber Temple may simply be incorrect despite his confidence in the
accuracy of the one thing he remembers.
