It is true that the argument for evolution is an inference to the best explanation, as most of science is, and thus it does seem a bit overstating it to call it a proof, which suggests deductive certainty based on undeniable premises. Calling it a demonstration is at least ambiguous between calling it a proof and treating it as if you can simply observe speciation in the past when you're not there, both of which would be a little misleading. So I do agree that it should be reworded. Demonstration is a bit strong for most scientific theories, perhaps with the exception of outright laws, which are fairly rare in science.