The challenge itself it's not bad, it's pretty standard. The problem is the lack of clear expectations about the outcome, the inconsistencies in the instructions (responses of endpoints that are not defined and you have to guess testing against the pipeline), the test examples that you let in the Java template do not work against your CI.
Also if you have non-functional requirements, it would be great to be explicit about them: Performance constraints? Concurrency? How many users?
The feedback it's really poor: it's like a template, you don't relate your comments against the code.
I have to say that all the comments on Glassdoor are true, this exercise was a waste of my time, you should truly review it to be more clear about the expectations, what is the focus on your evaluation and undoubtedly the feedback that you send.
I think that this is the worst evaluation process that I've done in my career and hope you really review it because it shows a really bad image about the Engineering area of your company.
For anyone reading this: if they keep with the Car Pooling challenge, if you value your time, you should avoid this company.