Is there an invisible wall above a certain building in the Metro Kingdom?
Yes, there is an invisible wall at that location.
A recent video posted on Reddit clearly shows Mario wall sliding, then diving into an invisible wall above the building in question.
Also, as Fredy31 suggested in the comments, I have returned to the area (after completing the main story) and Cappy confirmed that there is nothing else to find in the area, so there can be no Power Moons or Purple Coins above that building.