“no in-built cognitive biases.” isn’t quite right. For one, cognitive biases are sort of a matter of design, design limitations, and just plain wants.
Biases generally only get named as such when they’re malign, or benign but noticeable. The tendency to continue investing when further investment is neutral or disadvantageous has bitten a lot of people in the ass, so it’s been named the gamblers fallacy.
But there’s also the tendency to invest time and money into offspring, even when you don’t get any sort of time or money back, and even when said child does things you disagree with. And that’s never really termed as a bias.
Put simply, there are some things about intelligences that will simply be arbitrary. They want things, and they want them simply because they want them; there’s not really a base reason, or at least whatever reason was originally there no longer applies (and they don’t care). For humans and heavily-modified offshoots like the eldrae, they like music and tasty food, they laugh and they’re curious, and they have sympathy and love. All of these things can be rather inconvenient from time to time just like named biases, but they keep them around because they want them, or rather they are wants.
They tried making a AI without any of these base wants at a time. It worked perfectly… and did absolutely nothing: The Four Unlaws | The Eldraeverse
They’re still not quite sure if that qualifies as a mental illness worthy of intervention.
Now, as for what the eldrae don’t have, that’s more complicated. Keep in mind, they were originally modified by precursors to be, essentially, highly autonomous slaves capable of maintaining complex machinery and performing other difficult tasks. As such they nixed all sorts of things that would impact rational decision-making, along with a lot else. There are a dozen articles on the ‘lot else’, but in short it didn’t seem to affect child-rearing much. Also, it’s perfectly possible that the precursors made the babies less irritating as well.