Every time you see it its paired with a date: “Only In Theaters September 9th”. Why would they advertise that their movie is only in theaters for one day? I understand FOMO but that seems really intense.
I realize that the text is actually saying that it is running exclusively in theaters for awhile and that theater time starts on September 9th but the wording simply isn’t there…
“Only in theaters” is a separate statement from “September 9th.” One is stating where the movie can be seen, and the other if stating when the movie will be released. Neither is stating when or if the movie will no longer be available.
I feel like, back in the day, they used to say “in theaters September 9th,” which basically meant “[this movie will be] in theaters [starting on] September 9th.” But then with the rise of streaming they wanted to make clear that the movie would not be released on streaming initially, so they awkwardly tagged an “only” to the beginning of the statement. You’re right that the “only” was meant to modify “in theaters,” and not the date, but I don’t remember that construction independently before streaming.