LINQ is much more sophisticated, translating as much of the query as possible to the target (server) language, like SQL. JAQT is not doing that. lodash, underscore, and others, had a great impact on javascript by introducing lots of functional paradigms. Most of that is now part of javascript itself. JAQT's main purpose is to allow you to filter arrays of objects and create custom objects, just like GraphQL. And be a bit easier to learn than Array.map/reduce/filter.
Author here, if you are comfortable with filter/map/reduce, than JAQT doesn't provide much benefits, I agree. My target audience is developers who aren't that used to that. I've had much more success with junior developers using JAQT, then trying to get them to use map/reduce.
The target audience for unit tests is not the client, it is the developer. Unit tests allow you to change code with more confidence. 100% code coverage is not a useful aim, you should aim for 100% confidence in your code. Unit tests can also function as example code, that can't get out of date, since then the tests will fail.
Testing for quality assurance is a different thing, usually called acceptance testing and sometimes including regression testing. One easy way to make clients angry is by re-introducing bugs that were fixed in earlier releases.