I've seen similar. I wouldn't say 80%, but somewhere in the 20% range of people on phone screens. If I ask a similar modulo question that isn't obviously a rephrased FizzBuzz (i.e. "write a function that returns every 4th item in a list"), that percentage goes up dramatically. A lot of candidates memorize FizzBuzz without actually understanding what's happening and how to use it.
> This is self-contradictory: how/why would any open source project let someone that doesn't know fizzbuzz contribute to their codebase? I run a tiny throwaway open source project (~600 GH stars) and even my reviews are pretty stringent. This point of view is not consistent. It's like saying "I know people that ran in marathons, but couldn't even run for half a mile in my interview."
There are no time constraints, and no way to tell how much of their contribution is from their own knowledge and how much is copied from StackOverflow/other open source projects/etc. The point of view is consistent; I can't run a marathon, but I can walk 26 miles. That's fine for the local charity marathon, but it's not going to cut it if I want to do competitive marathons. The constraints and expectations are wildly different.