" they said that for those who use Prolog in production it is a real competitive advantage that they don't want to announce to the world;"

with due respect to prolog (which is a mindbending language), the proponents of every niche language say the same thing. "it is so powerful that users don't talk about it and that is why you don't hear about it"

I've heard many variants of this with respect to Forth, for example.

The reality is that devs can't stop talking about the languages, tools, and frameworks they use.

The simpler explanation is that next to no one actually uses prolog in productions, because it is, well, a niche language (which doesn't take away from its coolness)

Color me skeptical about "secret weapons".