The article makes an interesting speculation, but it doesn't pass it as a definite truth. There's nothing wrong with speculating as long as you know and tell the difference.
Scientists speculate and theorize all the time. They only get in "trouble", philosophically speaking, when they start thinking that a speculative idea is actually "real".
The same applies here: I have no doubts about the facts presented, and neither do you, and the theory even makes sense. But it's a theory and presented as such, in other words it's a "maybe".
We can't disprove the theory that "maybe" Van Gogh was color blind. We could disprove, or prove, that Van Gogh was indeed color blind, but no one is claiming that. The article is simply speculating that it was based on circumstantial evidence.