Neither Mythos nor Gemini are open-weight models (right?) so I'm not sure what model(s) this is actually referring to.

I'm also curious if they used Mythos. Google definitely had access to it via Project Glasswing. However, your logic doesn't hold. That sentence only tells us they were using multiple models, but offers no real clues as to which proprietary model(s) they were using.

Weird, why would they use open weight models.