Except it's not and =, it's a ≍ sign, which looks almost identical. I'm not a mathematician and I don't know the deep theory behind functional programming, but the internet tells me that ≍ in Graham, Knuth, and Patashnik's Concrete Mathematics it's defined to mean the same thing as "Big Θ"[1], as you note.
Normally, `f ≍ g` means `f \in \Theta(g)`. Here, the author is using it as an equals sign. I have no idea why.