Single press and swipe right on the iPhone seems intuitive enough to me. The issue I have with the current shortcuts (using buttons) for photos in every phone I've tested is that the pressing of the button is 'stiff' enough it involves jerking the camera slightly, leading to blurrier photos than using the screen tap. Also, obviously, volume button (which tends to be the shortcut key...) is on the wrong side of the phone, generally.

Hopefully this special button is a lot less 'stiff' compared to the other buttons.

Most likely it is capacitive and doesn’t actually move and they are using the haptics engine to make it feel like a button. It works surprisingly well on the iPhone SE with Touch ID. You can’t even really tell that it’s not actually moving when you press it.

In the side shot cutaways you could see an actual physical micro switch. Their cutaway shots tend to be accurate, so it looks to be an actual physical switch.

turns out it's both! seems like a physical microswitch for taking a photo with the button itself also being capacitive and they are using haptics for the "half press" feature (as well as touch sensitivity, slide between modes, etc).

Well, but it’s not both. It has a physical travel, that was the question. Whether there’s additional functionality beyond that done by cap/force was pretty clearly a yes based on the keynote.