An "overlay" is "a horse whose odds are greater than its potential to win," which is to say, a pretty good bet. (Looking up horse racing and gambling terms was quite necessary.) Guess betting on humanity's self-destructive irrationality is pretty safe too.

The text read as if it were an early draft, with many phrases and images repeated, some flabby bits; it feels near improvised, and the frame of having an alien author writing this—for his superiors? His peers in the Agency?—half excuses this. (Further, I think the Kindle edition I read was scanned and not corrected closely: there were many typos, missing punctuation, etc.)

Read it because I heard of it on SF Ultra with Sean McTiernan and Matt (from Bookpilled), for what that's worth.