Photochemical reduction of benzophenone: why inverted flask?
I think the reason for inverting the the round flask is to capture sunlight very efficiently. A round bottomed flask filled with a fluid is like a ball lens. Sun rays can be considered parallel and see how they focus. If light is coming from all directions, every part will be illuminated well as the Sun moves. See ball lenses here ball lenses

The clue comes from the instruction in the procedure given in your link &quot;Then the flask is exposed to the best available sunlight. &quot;