No, the timing as written is definitely off. I have to think that either Snape returned after finding out that the Potters were the target, but before their deaths (prophecy comes before Harry's birth, they aren't killed for another fifteen months) or that Dumbledore is allowing Harry to believe that remorse over the Potters is why Snape returned, while the truth is something else. I would go with Door #2, because it seems to me Snape must have had a better reason, and done something more than we know, to return and to have D trust him so implicitly.
But that's a good catch, it's definitely written oddly. Maybe she wants to draw attention to how illogical that train of thought is...
I had some kind of crazy idea that Snape isn't the same Snape. Maybe polyjuice potion and something to do with Regulus Black? But I'm not sure you can take polyjuice potion to impersonate someone who is dead, and there are no indications of Snape drinking anything on a regular basis.