That's what I was thinking unless they got away for a while and assumed another Star Destroyer picked them up. Anyways, I don't think she should have been at the battle, all signs in ANH point to her just receiving the plans. Again I would have had the plans transmitting to her a bit away from the battle, she gets going - light speed fails or something and boom a ANH. It bothers me that nobody producing the movie thinks about the beginning of ANH here.
So the other option is a bit of time passed and they caught up with her. She didn't know Vader had slaughtered a bunch of people which is strange because they were running the Death Star plans relay to get them to her.
I kind of wish they would have taken some cues from the radio drama. I always thought they did a pretty decent job of explaining what happened just before what we see in the movie. It also would have explained the lines about it being a consular ship, mercy missions, plans being beamed aboard, etc. better.
Although it probably wouldn't have made for a very entertaining 2 hour war movie.