I think you have your cause and effect backwards. Being paid 50k doesn't make you a bad developer. But bad developers tend to get paid less. At least, one would hope...
Developer pay seems to be mostly based on "years worked", not on merit.
That depends heavily on who the developer in question works for.
pretty much. the only time you can really use income to try and glean the quality of a developer is if they have alot of years of experience but still aren't being paid very much.
Location, location, location matters too. $50k in Alabama is at least $100k in SF.
Also, that number seriously depends on market. I work in a place that's absolutely lousy for coders and 50k is pretty typical here. I'd move if I hadn't put down deep roots.
After rereading that, I think all he is saying is that $50k is the floor. In other words, if you can get in the door you're making $50k no questions asked.