To me, however, the quote makes no sense. How can Kreia, even as knowledgeable as she is, know exactly how good they are? Furthermore, Avellone didn't create those Sith Lords, so I wouldn't be so quick to take his word as absolutely true. This is not to say they aren't skillful, but the quote seems hyperbole to me, especially because Exar Kun is not an ancient Sith compared to Sadow and Kressh (he lived 30 or 40 years before Kotor 2), and yet he is better than them, both with the Force and with the lightsaber.
The quote is of course hyperbole, but she wasn't referring to Exar Kun, she was referring to the Exile and herself. She's obviously not impressive herself, but the Exile is, and one could argue that this is more authorial intent than anything else, although, as you said, Avellone didn't create the Sith Lords (although he did expand on Tulak Hord, at least).