I would assume that, before the game starts, the imperial player starts with 2x the threat level (instead of zero) and can optionally deploy a group at one of the green locations. Is this correct?

Thanks a lot.

Yes.

Additional information:

https://community.fantasyflightgames.com/topic/216203-what-does-it-mean-to-increase-threat-by-twice-the-threat-level/