His original point was "all things being equal."
True, most writers can't write well.
If someone is good enough, though, from the ranks of fanfic and they get a job at Marvel, then what they are doing is no longer fanfic, but actual work.
If you are saying that So-and-So is a fanfic writer and what you mean is that they write at a level of bad writers everywhere, then you're case is true. But if you're saying that a writer is the same level of just a good fanfic writer, then Fused is right to say that the difference is in the pay.
It's all semantics, baby.
