> I believe it uses Prolog rather than Datalog (I'd be interested to learn more about how they differ).
Datalog is the positive, function-free fragment of Prolog, i.e., what Prolog offers over Datalog is negation and “value invention” through function terms. One consequence of that is that deciding whether a Prolog program entails a fact is undecidable (i.e., Prolog is Turing-complete), deciding whether a Datalog program entails a fact is can be done in polynomial time with respect to the size of the database.
It seems polonius is using datafrog instead. Where do you see a differential engine referenced?
The post linked from the readme mentions it: https://smallcultfollowing.com/babysteps/blog/2018/04/27/an-... , although it's two years old at this point so I wouldn't be surprised if things have changed.
It's not the 2 years that caused the change. Feel free to read up on Frank's blog post about how datafrog came into existence: https://github.com/frankmcsherry/blog/blob/master/posts/2018...
The Rust project is also using the differential-datalog library mentioned in the OP to underlie their third-generation borrow checker: https://github.com/rust-lang/polonius