I believe it would be possible for an expert C++ programmer (and this is clearly who they're targeting) to write an essentially compliant ("fully" sounds difficult, and I expect some fudge there) compiler in a year; two, perhaps, if it's a nights and weekends effort. This estimate comes straight out of my bum. It's an intuition. I'm not including the standard library: that would be a multi-year job even for a small team of truly excellent C++ programmers. But if we focus on the compiler, and allow a little fudge in compliancy, I think there are some individuals who could do it in a year. So I don't believe the scope of the task implies a joke.
The lack of information on the site raises worries. Do the organizers lack the confidence to reveal themselves? Or are they scamming? They shouldn't leave this a question.