Very thoughtful response and I agree technical reasons are to blame for Dolby itself recommending headsets to hear Atmos. But isn't that just another way to say the same thing? Dolby says headphones are better at SHOWCASING a billion dollar product. Believe them.
As I said, I don't think that putting that on a web page that is likely being consumed on phone or PC is Dolby saying what you think they are. My thought is that it's to get people to not just play it on the phone or laptop speakers and coming away thinking "what's the big deal ?".