The track pad itself is a button that can be clicked, and the up down left and right positions are pretty intuitive for taps and swipes (they can even automatically remap depending on the orientation of the remote). It's not enough to satisfy everyone, but developers definitely have more than "one button" to work with.
I think the "one button" concern already takes into account the given that the trackpad is for directional controls. It's the "other" buttons that will be a problem for a ton of games. I mean moving and "shooting" or anything else with the same thumb is going to be pretty awkward and then there's only one button with your other hand that's usable.
The rule makes sense on the iPhone- no game can require a 3rd party controller. Because the iPhone can emulate, at least awkwardly, anything on a console controller. Any game you can imagine controlling with a controller you can pretty much do on a phone screen (analog "stick" with multiple buttons or context aware buttons to compensate for the lack of physical ones). The Siri Remote is just a different input device entirely and will limit drastically the types of games that can be released for the TV.