Yes, it looks like a flash was used. A pyrotechnic "big chemical flash" was the standard kind in 1921, so that too.

I am not sure if it was "bounced against a wall to soften" or not, I don't think that our experience about what an electric flash looks like with and without bounce will apply, the pyrotechnic flash won't look exactly the same. The pyrotechnic won't be such a point light source for a start. So I wouldn't leap to the conclusion that there has to be a deliberate bounce.

From a balcony. I don't think Morey bounced it. Yes, magnesium powder I assume. No flashbulbs until a few years after 1921.