I wonder what it was that the FFXV team sent him since it's what sparked Tattsun's initial posts to begin with. Some of the FFXV staff replied to his posts saying they were glad it arrived to him safely, and one of the FFXV staff Toshiyuki Momose said to Tattsun that "I also think it would be good if someday the time for when we can talk about various things arrives."
So maybe it is something related to an NDA almost expiring?
Additionally this also may be nothing but FFXV dev Yusuke Tanaka also RT'd Tattsun's post but didn't directly reply, but he then posted this image of the Tokyo Metropolitan Government Building afterwards and then followed up by saying "Feels like there is signing ceremony".
Shimomura also just posted a video of Benyamin Nuss playing a new arrangement of Somnus.
https://twitter.com/midiplex/status/1269940498770194432
https://youtu.be/b4tiju6b0jg
She also posted one of Ivan Linn playing his arrangement of Veiled in Black a few weeks ago.
https://twitter.com/midiplex/status/1264139973881589760
https://youtu.be/7A6M4MHALq0
She's been posting various piano arrangements of her compositions recently to her twitter and youtube so it's nothing specific to just FFXV.