I see this debate a lot, and it makes me really quite sad.
A large part of the English language is in leaving implicit information out of dialogue because its unrequited. Obi Wan and Vader's exchange on that planet was about choosing a side, morality, and righteousness.
He was in NO WAY stating that 'only' the Sith deal in 'Absolutes' for everything. He was meaning that it was the Sith position, not the Jedi position that you were an ally or an enemy. That there was a 'right' way to do anything. The Sith believe that the power of the darkside is absolute and incontrovertible, but the Jedi believe in a living, changing force and that the only true power is in a balance of both.
He was implying that although he could not agree with Vaders point of view, he still (barely) held some love and affection for him - which was subsequently let go when he realised how far Vader had really fallen.
The statement makes perfect sense, unless read on its own as a standalone statement to the universe. And in those circumstances Obi Wan would of course never have said it.
What you have done is essentially as follows:
"Where are you going?
"The shops what?"
"I'm going to the shops..."
"Well why didn't you say?"
"Uh, I did."
You've completely pretended the context didn't exist.
That has to be the most eloquent way I have ever seen it explained. That's for this.