The mockup looks pretty in terms of how the overlay displayed but I think the Siri button in the menu bar is an annoying default. Gurman doesn’t specify, but I’m assuming it will be a default icon that can be removed (hold ⌘ and drag with the mouse, like other menu items). If it is fixed in place, like the Spotlight icon is on El Capitan … well that would really suck.
Activation through ‘Hey Siri’ or a keyboard shortcut should suffice. Note how on iOS, there is no persistent button or Siri app visible in the interface — you can only activate with voice or the Home Button long press. If Apple really does want an onscreen indicator, I think they should incorporate it into the transient Spotlight window. Although Siri and Spotlight are not the same, they do overlap a lot in functionality, so connecting the features together makes sense. It’s hard to justify two permanent fixtures in the menubar when the features share so much common ground.