I think that most people totally misinterpret this, but that also this and other aspects of the PT (Jar-Jar/"Icky Icky Goo!", Palpatine/"Unlimited Powah!", Anakin/"I hate sand", etc. etc.) continually remaining within the cultural conscious shows that the Prequels, despite them being 'bad films' by certain metrics, are actually brilliant films in ways that aren't really plainly obvious.
Obi-Wan saying "Only a Sith deals in absolutes." highlights the Jedi's hubris and the Jedi's hubris is exactly the environment that facilitates their and the Republic's destruction.
Obi-Wan even comes out of Padme's ship on Mustafar and folds his arms in a show of 'moral superiority' to Anakin and this only tilts him even more and reinforces his position that he'd been twisted into by Palpatine. It's even reinforced at the end "I have the high ground!" -> "Don't try it!".