That's designing it completely. In the interview, you're not expected to produce the final result; more importantly, it's not even about the result, but about the thought process and conversation.
Some (big tech) companies tell you to prepare for the test, because they try to standardize the system deaign open ended question. They end up selecting those that dedicated weeks of time learning for their version of the test. For the others it's a psychological game trying to predict what is on the mind of the person asking the questions.
The bigger the company, the more likely people have "trained" up to try and give themselves an advantage over others. I don't think companies are necessarily looking for people who have dedicated weeks of learning, but that learning can certainly make the applicant look smarter than they might be.
Other employers might expect someone who is "Senior" to know about what different languages offer. As others have said, the title "Senior" means different things to different people and not all skills are transferable.
As others have said, if they don't do interviews well, don't sweat it, you probably don't want to work there anyway!