It's especially strange why they're calling it a language on their blog when the GitHub readme specifically states that "Pyro is a flexible, scalable deep probabilistic programming library"[1]

1: https://github.com/uber/pyro

From skimming the documentation it sounds like, rather than being just another library of utility functions, there's a formal language of sorts under the hood that the programming library "compiles to":

Think of this as the compiler for a PPL: it allows us to divide labor between the modeler and the inference expert.

Or something like that. Someone from the team could no doubt produce a better explanation.

Maybe the idea is that it's a DSL?

I was going to suggest that the author was in marketing and didn't know the difference, but the article seems too in-depth for that. Maybe it's hybrid and devs wrote the more technical sections.

This bit [1] seems to be hacking around with the Python runtime to provide "DSL" like semantics. I wouldn't call this a PL but a DSL, sure. (as an aside, reading this "Poutine" stuff everywhere made me super hungry)

[1]: https://github.com/uber/pyro/tree/dev/pyro/poutine