In a real-world scenario, you meet and/or communicate with the client about their spec to confirm and clarify that you understand what they are asking for. After these issues have been addressed--in a real-world scenario--you often even enter into a clear contract with said client about precisely what will be delivered.

Yes, this problem was badly worded and it wasn't hard to figure out what they wanted based on the given examples. Yes, developing the ability to intuit what your client needs out of the mess of what they think they need is an important life skill; however, this is not real life. This is a website that markets themselves as a tool for teaching, training, and exercising code skills. The exercises on this website are a professional business product, and the users act as the client. There is no mode of instant contact with the problem's poster. It is more than reasonable for users to expect concise problem descriptions and accurate solution testing, just as you would expect from exercises in a textbook.