My guess is that it's deliberate - that Siri window hovers over all apps so that you remain in your current context, and can add files/images/texts from any app to the Siri conversation. There might also be a Siri app on the Mac that was not yet shown.
You're probably right, just add it to the collection of "not-quite-window floating thingies", accompanied by Quick Notes, Stickies, Special Character Picker, Color Picker, Font Picker…
Sigh, I wish we could stop re-inventing what was already solved 25 years ago.
Which means, if shipped like this, the Siri dialog will be a poor excuse for a window with:
- no Cmd+Tab, no Cmd+`
- no minimize??
- generally no presence in the Dock whatsoever
- no keyboard shortcuts beyond basic text editing ones
- no smart window resize
- …
So in other words, no Justin, that’s not a window. That’s a resizable Spotlight pop-up with an “X” button.