The message isn't "the rapture already happened", the message is, the day of Christ/Lord is here, with "here" to be understood as, it's come, and it's staying. That day, it's come, to stay. It's in the perfect tense, that's what that tense means.
This is one reason why your interpretation simply doesn't fit with the wording and the syntax of the passage. Your interpretation has them being told, "Rapture day is here to stay", which doesn't make sense, considering the rapture is an event that occurs and then it's over. So these terms, "the gathering to our Lord"/"rapture"/harpadzo, and, "the day of Christ/day of the Lord" are not equivalent.
Again, this isn't from an interpetation, I'm relaying the syntax of the passage.
Much love!