You will want to be in your corral by 9:30-9:40 a.m. so work backward from there.
You'll be dropped off at athlete's village - a little less than one mile from the starting area and it's going to take a while to make that trip to the start because of the crowds. And the security checkpoints. Legitimately give yourself 15 minutes to make the trip and find yourself in your corral. (That also assumes you do a light jog to the corrals).
You'll probably want a little warmup too before you make your way to the corrals. Something where you're just running on a small uncrowded street - so that's another 10 minutes (we're already at about 9:00 a.m. if we we are working backward).
When you're out in the ballfield, you'll probably want some food, drinks, use the bathroom (and battle the lines) so again, plan a solid 30+ minutes just for that. (8:30 a.m.)
It takes about one hour on the bus just to get out there from the moment it departs Boston Common, maybe longer. (7:30, at best).
And it takes a while just to wait in the line of runners and board the bus. 7:00 a.m.
So if they're telling you that it's a good idea to be in that line by 6:15 or 6:20. They're not wrong.
You've trained all winter for this. You're going to have to go through a bunch of crap just to get to the start. You might as well give yourself that extra 30-45 minute cushion to make sure you don't miss the race.