Having been a student of Art History myself, I’d say the only real evidence of the correct orientation of this painting is the photo of the painting on Mondrian’s easel. A lot of the work of 20th century abstract art has been subject to the interpretation of the gatekeepers of the meaning of modern art- gallery owners and academics, mainly- who stick doggedly to their self-designed orthodoxy to keep their exalted places.

Mondrian’s work is non-representational. He could have well been working in that orientation to make the work easier. So unless Mondrian himself said it, the “thickening at the top” to represent the sky is, when you look closely at it, just as much horse feathers as it is cogent analysis.