It doesn't "know" about the menu. When a touch event occurs we're in the native thread and need to decide whether it goes to.a peer or a lightweight.
So we loook through the component hierarchy to see if it's one or the other. There's no lightweight in the hierarchy at that stage so the browser component grabs the event and the lightweight menu doesn't get it.
The solution is a special case for the side menu swipe in the native OS ports. That's pretty hard to do so I'm not sure when we'll get around to do it.