How is B#°7 the vii7 of E?

[ClairDeLune]

Chopin's Op. 64 No. 2, the Waltz in C# minor, has an interesting analysis I found on YouTube. In measure 6, it states that it's in A major with a pedal point on E, starting with the chord E, F#, A, D#, and B#. I'm a bit puzzled because shouldn't the vii of E be a D# chord instead of B#? Any insights would be appreciated!