Type information generally isn’t used when doing property testing in Elixir, and Python has a very mature library too (hypothesis). If there’s a difference in popularity, it could just be culture. The Erlang/Elixir community is rather fanatical about reliability.

Hypothesis uses the same type annotations as Python's optional typechecker as a tool to select appropriate input data generators without additional test-specific description. That's a fairly common feature of property-based testing tools; I don't know for sure, but I wouldn't be surprised to find that hypothesis and mypy tend to be used in the same segments of the Python community.