512 is not a maximum resolution for anything.
The height value of a heightmap is infinite.
You can set the z scale to whatever you want.
The end result is 1/65535 * resolution you picked = vertical distance between pixels of different colors in heightmap.
(Uu or centimeters I think).
If there really is bad verbiage stating a limit on height factor in the docs, then everyone should be filing a complaint with the doc editors for yet again writing bad documentation.