Sort of.
WDW resort guests can start at their 180 day mark for the first day of their vacation, and they can book reservations for up to 10 days AFTER that. So they can't book at 190 days from their first day...but they can book at 190 days from their 10th day.
Yes, offsite guests have a shot only at exactly 180 days from when they want to dine.
I don't know about how that affects BOG, which is a very large restaurant with a capacity of more than 500 guests. Yes, an onsite guest might get first shot.