It depends how you look at it. I don't remember where they've specifically said it was a letter, or where it was from Tifa except from English fansites.
In a way, there's a message from Aerith in the film, which was supposed to use elements from the original. It was a message to children, rather than to Cloud, though.
But there's so little known about the original version. Tifa being in it apparently means the message can't be from her, but we don't know the composition of it. Tifa might just appear at the end, after sending a message with kids. Cloud goes back to Tifa, and that's where she appears. Or it could be Cloud leaving Tifa, then getting a message and finding out it's from Aerith and going off for flower field lovin'. Or Cloud leaving Tifa, then getting a message, which sends him back to Tifa. Cloud and Tifa could both get the message, maybe Tifa coming in later, and find out it was from Aerith.
There's so many ways you could spin that vague outline of a plot.