Rippling Senior Backend Engineer interview questions

Interview process started well with recruiter call. It then went to a Technical interview question about implementing a cache class with concurrency/thread safety. Also went great. After that, I was told I passed to next stage but then shortly after was told that the position isn't available and it is now a frontend focused position available. I said I can do frontend as well. I then was ghosted. Like other software engineers have said, I don't think they actually have any positions available. They are guinea pigging people to gather data for some process. There are multiple people with the same experience in Glassdoor