Does ferrous/non-ferrous matter for ground planes?
Both your provided links sound like they were written by someone with very little understanding of how antennas work. I would ignore them entirely.

A ground plane is a necessary part of a monopole antenna, often called a "vertical" since that's how they are usually mounted. These antennas are essentially half a dipole, with the other half being the electrical "mirror" of the antenna on the other side of the ground plane.

The only requirement for the ground plane is that it conducts. The more conductive it is, the better. The bigger it is, the better. Professional AM radio broadcast antennas use lengths of copper wires buried just under the ground, radiating out from the base of the antenna, to make the Earth more like an ideal ground plane. I'm sure if iron worked better, they'd use iron, as it's much cheaper than copper.

If I had to guess, the reason CB operators think ferrous metals make better ground planes are two-fold:


they have no clue about how antennas work
magnet-mount antennas, very popular for CB, won't stick to non-ferrous metals (though other than not sticking, they work fine)


If you don't actually have an antenna, then none of this applies. The lesson to be learned is this:

Don't use references written by CB operators as a source for learning about electrical engineering of any sort.