Kreia is the opposite of reputable. Even when she didn't actually lie she'd often say something nuts.
That said, I don't really have a problem with her saying "Revan was power." But all that really means is that Revan was powerful. Which we already know. That doesn't mean he was the most powerful Sith out of the 4 of them.
Kriea is like Sidious but in a different time period. She wasn't all that powerful (contrary to Sidious), but she could manipulate many people to do her bidding. But the thing is, she always lies. She told Sion that he would be her right hand (since she lost it to him
), but that was a lie. She always wanted the Exile.
That quote really shows how badly she lies. I look at it as a way to push the Exile to get stronger. "Revan was my greatest student. You suck compared to him!" It was really meant to make the Exile stronger so Kreia's plans could come to fruition.
But it doesn't change the fact that these three other Sith Lords are more powerful than he is.