Only the Sith deal in absolutes?

Although I think the prequels make a great case by showing all the reasons The Jedi can fail and in part it DOES relate to becoming too rigid, there is still a difference in degrees of absolution via philosophical motivation and methodology.

Yoda: The council is confident in its decision, Obi-Wan.

Mace Windu: The boy has exceptional skills.

Obi-Wan: But he still has much to learn, Master. His abilities have made him... well arrogant.

Yoda: Yes. Yes. A flaw more and more common among Jedi. Too sure of themselves they are. Even the older, more experienced ones.

The Jedi were suppose to be peace-keepers and negotiators of the Galaxy during the time of the prequels. Their philosophy, as laid out by Anakin in Attack of Clones, leaned towards what we may perceive as Buddhism, where the idea is not to be materially attached to objects and other people, but rather find compassion and be encouraged to more selflessly love.

Anakin: Attachment is forbidden. Possession is forbidden. Compassion, which I would define as unconditional love, is essential to a Jedi's life. So you might say, that we are encouraged to love.

Now granted the notion of raising younglings, brotherly/sisterly training Padawans, having a hierarchy with The Jedi Council, and notion of lighsabers ("This weapon is your life!" - Obi-Wan to Anakin) proves that the Jedi COULD NOT BE ABSOLUTE in their own philosophy, but there is a sect of Jedi called Ordu Aspectu that might have been and depending on whose story one believes, one can easily see their absolution made them closer to the Sith.

Obi-Wan: I have to admit that without the clones, it would have not been a victory.

Yoda: Victory? Victory you say? Master Obi-Wan, not victory. The shroud of the dark side has fallen. Begun the Clone War has.

But on top of that The Jedi also had committed to work in tangent to The Republic and it is that commitment that turns them from peace-keepers into soldiers for the Republic.

According to one account, supported by the father of the archaeologist Aphra, the Ordu Aspectu was a violence-loathing Jedi sect that sought to selflessly prolong life for all. When the orthodox Jedi raided their fortress, the Ordu Aspectu were forced to activate a device that seemingly caused them to disappear, perhaps ascending to a higher form of existence. However, according to the younger Aphra, another version of probable events was that the Ordu Aspectu had kidnapped Jedi Padawan learners, whom they callously sacrificed in an attempt to gain the immortality they sought. Aphra also stated that the Ordu Aspectu might not have existed at all, and the term might simply have been an argument between Jedi grammarians of more recent years.

But when we go back to Ordu Aspectu and the idea that their introduction into Star Wars cannon is really again about contesting the mainstream Jedi philosophy, the reason they are possibly more like the Sith, is because they lack compassion...

and it's the lack of compassion and the pursuit of the self over caring for others that the Jedi would view the Sith ways as more rigid and absolute.

Specifically the line you refer to, is about Anakin believing that there is only good and evil and as Anakin explains, he now sees the Jedi as evil and Obi-Wan disagrees with a "good vs evil" absolute way of thinking.

Speculatively, Anakin makes a lot of great points when one considers how scared even Yoda was of him at first, not wanting to train him, because of his own fears of Anakin's fears, and not by being able to see Anakin's future (The Temple was unknowingly built on top of a Sith Shrine so geological metaphysics also plays a role here), but in the same breath, if you go back to the first block of dialogue I provided above, Yoda specifically "changes his tune" from the first film to the second.

In fact each prequel films shows Obi-Wan, Yoda, and Mace Windu changing their feelings regarding Anakin. Each film pits one of those three against him. Yoda in The Phantom Menace, Obi-Wan in Attack of the Clones, and Mace Windu in Revenge of the Sith all contest and admit to not trusting Anakin at different times, while in others, they are supportive. This also shows "flexibility" and not absolutism, despite that the continious mistrust coming from someone, helps to push Anakin into a more self-absorbed absolutist view...

or maybe not...In the current Darth Vader comics taking place shortly after Revenge of the Sith, readers are learning that the transformation into Darth Vader was a struggle and although Anakin conitnued to do a lot of bad things for the sake of staying alive and power, the reasons behind it for him were still about love and pursing paths that may allow him to save Padme from death, showing us human frailty and a need for redemption.