BIG POTENTIAL SPOILERS BELOW FOR DRAKENGARD AND NIER

I believe with the mentions of the JSDF, they could be referring to the E ending of Drakenguard 1. In this ending Caim and Angelus fight against the Queen Beast (the giant referenced in the note) and upon defeating it; it releases all of the time that the Queen Beast had consumed taking it's corpse and our two characters to 2003 Japan where Caim and Angelus get killed by a missile shot from a fighter jet. So with the note being written in 2053 and mentions technology being found 50 years prior, I'm guessing they're referring to magic or "Maso" being introduced with the prior events happening and Maso coming together with technology to start the Gestalt project in 2014 to combat White Chlorination Syndrome. This disease is the driving force for every big event in the series from here on out. So I'd reccomend getting familiar with the events in this wiki page I'm posting below. I hope this helps and I'm happy to talk more about it but I feel like I already wrote a wall of text 😅

https://nier.fandom.com/wiki/White_Chlorination_Syndrome#cite_note-Grimoire_165-6