I would say that someone could have the required level but not the required exam passing skills, and therefore they might fail the exam. I don't think someone could pass the exam without having a solid level in the language (no matter how well they know the exam structure, tasks etc.).
Also, there's nothing wrong with guessing our approximate level, is there? The C1 French exam was my first ever language exam, and I guessed (from my own self-evaluation) that that would be the appropriate level exam to aim for. Guessing one's level in a language is kind of fun, and motivating, IMO.