I'm struggling to see why it's supposedly better to make it hard for children to see the unadorned truth through all the mythology instead of just telling them the truth in the first place

There's so much in that sentence! But the biggest problem with it is that you set out with the view that Pete and co are deliberately lying to their children. They are not. They are telling what they believe to be the truth. Equally, they are convinced that if they taught evolution as 'the truth' that would be a lie. I fall into a different camp: I see evolution as true (to the extent any scientific theory is ever sensibly labelled as 'true') but there are other ways of looking at the world that do not invalidate that truth.

I can only refer to the anecdotal account of my own children, but they were a damn sight more sophisticated at five than you seem to give children credit for! They had (children's versions of)some of Shakespeare's history plays which they loved and were quite able to appreciate them as stories at one level, and historical true and historically false at parts at another. In a child's way, of course, but it is not to be trivialised.