The significance is she had no flower on her mailbox before in other iterations of this scene. This is the same flower Cloud gave Tifa in FFVII Remake Chapter 3 as a gift to celebrate their reunion, and now is on her mailbox in his own recollection of her house.
Cloud’s memory of these 10 days shown in the game are a culmination of Jenova muddling his true memories with Zack’s and other altered ones in his head, which means this is either a true detail Cloud remembers about her house, or a false detail Cloud has attached to Tifa in his mind, which either means he specifically associates Tifa with that flower.
The only other place it shows up on her mailbox is during Tifa’s trial. Fun fact: Only Cloud and Tifa both show up in each other’s trials in Temple of the Ancients. This scene is also a type of physically manifested mental realm, so the fact that the flower only shows up for the two of them is an interesting new detail added for Rebirth.
