But notice that this isn't evolutionarily stable. If a mutation causes a smart player to start defecting in non-public rounds, then it would have an advantage.

Evolutionary stability isn't about TDT because organisms don't simulate each other. You, however, are running a very small and simple computation in your own mind when you conclude "smart players should defect on non-public rounds". But this is assuming the smart player is calculating in a way that doesn't take into account your simple simulation of them, and your corresponding reaction. So you are not using TDT in your own head here, you are simulating a "smart" CDT decision agent - and CDT agents can indeed be harmed by increased knowledge or intelligence, like being told on which rounds an Omega is filling a Newcomb box "after" rather than "before" their decision. TDT agents, however, win - unless you have mistaken beliefs about them that don't depend on their real actions, but that's a genuine fault in you rather than anything dependent on the TDT decision process; and you'll also suffer when the TDT agents calculate that you are not correctly computing what a TDT agent does, meaning your action is not in fact dependent on the output of their computation.

TDT could never have arisen in the first place by evolution

It didn't.

Evolutionary biology built humans to have a sense of honor, which isn't the same thing, but reflects our ancestral inability to calculate the unobserved rounds with exactitude.

TDT can arise in many ways - e.g. a CDT agent who believes they will in the future face Newcomblike problems will self-modify to use TDT for all Newcomblike problems dependent on decisions made after the instant of CDT self-modification, i.e., "use TDT for problems dependent on my decision after 9am on Tuesday and CDT for all problems dependent on decisions before then". This is inelegant, and a simple application of the unknown meta-decision-theory that wakes up and realizes this is stupid, says "Just use TDT throughout". A true pure CDT agent would never realize this and would just end up with an ugly and awkward decision theory in descendants, which points up the importance of the meta-problem.

But evolutionary dynamics simply are not decision-theory dynamics. You might as well point out that no interstellar travel could arise by evolutionary biology because there's no incremental advantage to getting halfway to another solar system.