I think you're over-egging the pudding a bit here. If people on this board are allowed to discuss evolution, then everybody is. Some people are more informed than others and I would suggest that Prof Hawking falls into the first category.
And when Stephen Hawking decides to come onto this board and share his opinions about human evolution, he's welcome to do so. The thing is, as an astrophysicist by training and career, he is no more an authority on human evolution than anyone else on this board. It would be no different if Hawking were, say, the world's most successful car salesman or an incredibly famous accountant. There's nothing about being a PHYSICIST that makes you inherently better informed or even more intelligent on matters unrelated to physics.
The funny thing is, we recognize this in most cases. Physicists are treated as inherently mentally superior to everyone else, simply because physics involves math, and math is hard
. Evolution, AFAIK, doesn't involve a whole lot of math, and what little it DOES involve is of a very different type than used in physics.