> some form of RF mixer is required to achieve the effects described in the second to last paragraph
I think the author is implying that the mixing is done in the transmitting satellite and since most receiving equipment is DSP or SDR (both completely digital) these days the ELF signal would be undetected unless you knew to look for it.
It could be a workable scenario to produce the wanted 800mhz signal along with an otherwise undetected ELF signal.
The problem I see is that transmitting an ELF signal from above the ionosphere and expecting it to reach the surface is a bit far fetched.