The author of the article claims that it proves Evolution. A common process is used to perform multiple functions, and this is taken as evidence that the process must have started out having one function and later been co-opted to perform the other.
Where in the text does the author claim that it "proves evolution?" Please point me at that, because I missed it. At most this is simply a mechanism that is consistent with predictions an evolutionary biologist might make. No rigorous scientist would call that "proof." This writer didn't, and he didn't use your words "must have," either. In the context of the blog, this post is clearly intended as correlative evidence of evolution, but no one's calling it definitive. Your argument here is a strawman argument.
(And why are you capitalizing 'evolution' every time you use the word? It's not a proper noun.)
Now, if one starts with the preconception that Evolution is the origin of all functions, then naturally interpretation of the evidence will follow this preconception. This is the case with all evidence about all things. But if we start with no preconceptions, we see that this evidence really says nothing about Evolution one way or the other.
If you're walking in a field and see an acorn on the ground, and an oak tree a few yards away, then only your preconceptions might lead you to connect the acorn with the tree. If you have no preconceptions, the evidence of the acorn really says nothing about the tree one way or the other.
If you're walking in a forest, however, and see hundreds of trees and thousands of acorns upon the ground, and some of the trees have budding acorns upon them, then you really should feel justified in inferring a connection, even if you never see an acorn fall during the short time you're walking.
I do not dispute the facts. I dispute the interpretation.
Now let's say you're walking with a forestry expert, and the expert says "Acorns come from trees." You are not a forestry expert. You say, "Don't be ridiculous! Look, here's an acorn, and it's on the ground. It's not on a tree." The expert coughs politely and points out some acorns hanging on trees. You say, "What does that have to do with anything? I'm talking about
this acorn! Right here!
This one is on the ground! You can't prove it has anything to do with a tree!"
Each of these bits of evidence, sonic hedgehog and everything else, is an acorn. Are you going to inspect each one individually and say that it, alone, doesn't prove evolution? If you lift your eyes from ground level and observe that there are thousands of them, they're highly consistent with each other, and there are trees all around you, does that signify nothing?
More to the point, do you really think you're smarter and better informed than the sum of the world's biologists, practically all of whom believe that evolution is a fundamental and well-founded mechanism of their science, with a huge mass of observational evidence supporting it?
How sure are you that you're the clear and critical thinker in this game, and that the people who spend their lives studying this science are all banally misguided? What's your insight that they don't have? Given your self-confessed lack of scientific knowledge beyond the lay level, how confident are you in your superiority? And what's the rational basis for that confidence?