There was a clue waaaaay back on Ch10.1 page 7 about Nagatoro’s uncomfortable reaction to her martial arts. It seems this backstory was consistent since the first volume,
Okay, I'm not 100% certain, but that stance reminds me of Nagatoro's pre-serialized stance that Nanashi drew before it became serialized (Quite NSFW/NSFL [CH.3]: https://mangadex.org/title/23272/nagatoro-san-doujins)