I'm not sure they're wrong to be honest (assuming an average cross section of people). Rationality is an extremely long term approach and payoff, I am not sure it would even work for the majority of people and if it does I'm not sure if it reaches diminishing returns compared to other strategies. The introductory text (sequences) is 9,000 pages long and the supplementary texts (kahneman, ariely ect) take it up to 11,000. I'm considered a very fast reader and it took me 3 unemployed months of constant reading to get through. For a good period of that time I was getting a negative return, I became a worse person. It took a month after that to end up net positive. I don't want to harp on about unfair inherent advantages, but I just took a look at the survey results from last year and the lowest IQ was 124.6. This stuff could be totally ineffective for average people and we would have no way of knowing. Simply being told the best path for self improvement or effective action by someone who was a rationalist or just someone who knows what they're doing, a normal expert in whatever field may well be more optimal for a great many people. Essentially data-driven life coaching. I can't test this hypothesis one way or the other without attempting to teach an average person rationalism and I don't know if anyone has done that, nor how I would find out if they had.

So far as instrumental rationality not being in core about truth, to be honest I broadly agree with them. There may be a term in my utility function for truth but it is not a large term, not nearly so important as the term for helping humanity or the one for interesting diversions. I seek truth not as an end in itself, but because it is so damn useful for achieving other things I care about. If I were in a world where my ignorance would save a life with no downside while my knowledge had no longterm benefit then I would stay ignorant. If my ignorance was a large enough net benefit to me and others, I would keep it. In the arena of CEO compensation for example increased transparency leads to runaway competition between CEOs to have the highest salary, shafting everyone else. Sure, the truth is known but it has only made things worse. I'm fairly consequentialist like that.

Note that in this situation I'd still call for transparency on war crimes, torture and so on. The earlier the better. If a person knows that their actions will become known within 5 years and that it will effect them personally that somewhat constrains their actions against committing an atrocity. The people making the decisions obviously need accurate data to make said decisions with in all cases but the good or damage caused by the public availability of that data is another thing entirely. Living in a world where everyone was rationalists and the truth never caused problems would be nice, but that's the should-world not the is-world.

It so happens that in this world we live using these brains we have, seeking the truth and not being satisfied with a lie or a distortion is an extremely effective way to gain power over the world. With our current hardware truth seeking may be the best way to understand enough to get things done without self-deception, but seeking the truth itself is incidental to the real goal.