What about the fact these are all pictures of airports? Does that control for "any time it's not flat" or are you talking about really, really flat? Or, given your mention of angles, do you mean flat as in perpendicular to the observer?
With that said, raw elevation matters too, especially when the imagery is off-nadir.
RPCs describe a view direction + lens distortion / etc. A completely flat surface at elevation will be in a different location than a completely flat surface at sea level. Things will also be a different _size_ as well depending on elevation.
Think of the RPC as a vector looking a specific direction (at a single pixel level, that's kind of what it is, though each pixel gets a different one). You're placing a point on the map where that ray intersects the Earth's surface. It intersects at a different point depending on the elevation of the surface. Projected (aka orthorectified) data needs to correct all of that to be able to place imagery on a map accurately.
If the airport is completely flat, then using a constant elevation is fine, but you still need to use that elevation and not assume it's at sea level. Otherwise it will be in the wrong place on a map and also a slightly incorrect size.