Yes, but I think it achieves that by extracting the core out of WordPress and using that to power the other systems.
So
AFAIK, it's basically just the guts of WordPress without the fluff ... I think.
Then you can take that and build other stuff like bbPress with it and since they operate in much the same way under the hood, it should make getting them to work together fairly easy. It also allows the same code to be used for both projects which saves development time and should (in theory) reduce bugs.
I think it's basically a PHP framework of sorts, but aimed at creating CMS's in particular. I could be WAY wide of the mark though. I'm just taking pot-shot guesses here.