It’s just the way the iPhone / Safari works, I guess - in landscape orientation, if you have a non-scrollable web page, you’re stuck with the address and tab bars at the top of the screen.
Adding the Zap toolbar as well makes it even worse - you end up with a WebAR experience that’s super-wide and very thin.
I don’t think there’s a way Apple could gracefully hide that chrome at the top, as there’s no obvious way for them to know when the user wanted to display them. Scrolling webpages are different of course, and handle this automatically; scrolling down triggers the chrome to disappear, scroll up and it reappears. Wouldn’t work with WebAR, where swipes and touches need to be passed on to the content.
An alternate Zap toolbar for landscape mode would help a lot, though: if you put the buttons in a vertical strip at the side of the frame, on the left or right, it’d leave a much more natural and useful shaped region for the WebAR to fill.
Could I log this as a request please?