Just after a quick glance over the original post... I don't think the guy was talking about whether one would be able to understand it linguistically, but on a more fundamental level.
Having said that, I'm still trying to figure out to what degree one Arabic speaker can understand another. When I wanted to learn Arabic, I was often told "well... you have to decide which kind, because you won't be able to communicate outside of that dialect" It was almost like people were trying to convince me not to learn it. But then I see so many people who speak it, communcating across dialects. So whatever.
(as it turned out, I went with Spanish instead
)