Are Cognitive Biases Design Flaws?

I am a newbie so today I read the article by Eliezer Yudkowski "Your Strength As A Rationalist" which helped me understand the focus of LessWrong, but I respectfully disagreed with a line that is written in the last paragraph:

It is a design flaw in human cognition...

So this was my comment in the article's comment section which I bring here for discussion:

Since I think evolution makes us quite fit to our current environment I don't think cognitive biases are design flaws, in the above example you imply that even if you had the information available to guess the truth, your guess was another one and it was false, therefore you experienced a flaw in your cognition.

My hypotheses is that reaching the truth or communicating it in the IRC may have not been the end objective of your cognitive process, in this case just to dismiss the issue as something that was not important anyway "so move on and stop wasting resources in this discussion" was maybe the "biological" objective and as such it should be correct, not a flaw.

If the above is true then all cognitive bias, simplistic heuristics, fallacies, and dark arts are good since we have conducted our lives for 200,000 years according to these and we are alive and kicking.

Rationality and our search to be LessWrong, which I support, may be tools we are developing to evolve in our competitive ability within our species, but not a "correction" of something that is wrong in our design.

Edit 1: I realize there is change in the environment and that may make some of our cognitive biases, which were useful in the past, to be obsolete. If the word "flaw" is also applicable to describe something that is obsolete then I was wrong above. If not, I prefer the word obsolete to characterize cognitive biases that are no longer functional for our preservation.

It is a design flaw in human cognition...

So this was my comment in the article's comment section which I bring here for discussion:

Since I think evolution makes us quite fit to our current environment I don't think cognitive biases are design flaws, in the above example you imply that even if you had the information available to guess the truth, your guess was another one and it was false, therefore you experienced a flaw in your cognition.

My hypotheses is that reaching the truth or communicating it in the IRC may have not been the end objective of your cognitive process, in this case just to dismiss the issue as something that was not important anyway "so move on and stop wasting resources in this discussion" was maybe the "biological" objective and as such it should be correct, not a flaw.

If the above is true then all cognitive bias, simplistic heuristics, fallacies, and dark arts are good since we have conducted our lives for 200,000 years according to these and we are alive and kicking.

Rationality and our search to be LessWrong, which I support, may be tools we are developing to evolve in our competitive ability within our species, but not a "correction" of something that is wrong in our design.

I thought instrumental and epistemic rationality feed each other, no? Can you be one but not the other?

An epistemic rationalist is an instrumental rationalist who values truth. An instrumental rationalist values something else. If they value typical things like wealth and status, then there is some evidence that the winners in society have won by systematic winning. But epistemic rationalists don't often win in those terms.