Oh the fun of semantics, I love words.
My assumption is that: for you, being an athlete requires that someone be engaged in an athletic sport. That's fair if that's your definition, but it opens up a question for where the line is then drawn. Does the sport need to be professional? Competitive? And that then opens up the question about what you define as a sport, and perhaps what you define as an athletic-sport if you believe that some things are sports but not athletics (examples MIGHT include: hunting, racing cars, jockeys, bowling, golf, etc, etc).
I like to just keep my definition for athlete as being: anyone capable of doing athletic feats of some sorr. This way we can leave it broad and we just debate which athlete is more athletic than another (rather than whether or not someone is an athlete at all). My definition is certainly more broad, but I think it just makes defining an athlete alot simpler to do.
As far as Chris Hemsworth being an athlete, I don't really know. He may just be in terrific movie star shape, and he may only lift weights and eat right and deliver awesome performances on front of green-screens (calling in the stunt men for the athletic necessities of the character). But it would surprise me to find out that he does nothing (and is incapable) of doing any athletic feats.
(Btw, if you can't tell, I've had this debate more than a few times with folks that I know and it never gets old for me!)
I appreciate and value your opinion, so if I come across as rude or condescending I apologize for that, it's absolutely not my intention in any way.