So just to summarize what is written in the other post:
\paperheight were introduced by LaTeX (the old
dvips days). These options were kind of unimportant as there was still the DVI driver to configure the page size.
pdftex one had to setup page dimensions as required by the PDF format and the values were taken into the LaTeX format (as
\pdfpageheight and width). For a real change in dimensions you may adjust
\pdfpageheight (or width) later on; adjusting
\paperheight is insufficient. So the pdf thing was introduced to really change the output.
And therefore this has nothing to do with
xdvipdfmx, but XeTeX for instance understands the pdf commands for compatibility. And yes, as you could read above, there is a difference between the compilers.
The only question not answered in the other thread is the TikZ question. But why TikZ chooses one value over the other should be the question of compatibility to other engines. Especially those which do not respect the pdf commands.