Great stuff from that redditor.
Lon Shapiro

It made sense to me,

I think Arya was trying to let Sansa know what was up. Sansa clearly knows Arya had no desire to be the Lady of Winterfell etc. She gave her the dagger and showed her back to suggest she could trust her (symbolically). She didn’t say it explicitly because the walls have ears (Littlefinger has little birds too).