I disagree, it turns out that Mickey hasnt been back at Disney Castle since he left before Sora awakened. Which means he's been sending letters via Pluto home (my theory on why Pluto was at the end of KH with a letter)
So that was most likely just a letter to Minnie, i believe Mickey wrote the letter that they find at the end before he left.
But i also agree that KH3 does seem less urgent, but i dont think that ther letter was written before hand because, if it was, how did he get it back before going home to Disney Castle?