I’ve never used TaskPaper, but the capabilities of the data format look somewhat similar.
I think the difference is primarily a philosophical one: [x]it! is a file format with a formal specification that’s open source.[1] There is no “canonical” tool for it, the idea is rather that tools can be created separately. That should give users the freedom to work with their data independent of specific tools.
[1] https://github.com/jotaen/xit/blob/main/Specification.md