From jb:

Please demonstrate for us were and how this "ancillary", "ad hoc"
result is
tacked on to GR.

Simple. If your re-held belief is that space is "void", then
longitudinal pressure waves are impossible. So your math has to 'prove'
GWs are something other than longitudinal, just as Ptolemy's math
'proved' the Earth is the fixed center of the universe. Likewise math
can 'disprove' Newtonian gravity's instantaneous-ness. As mentoned
numerous times before, math can be used to "prove" virtually any
pre-held belief that's currently in vogue.

Seems you're pretty good at running your mouth and little more,
brainiac. So suppose you demonstrate for us, in your own words, and
without a hand-off to some website, why, if there is no carrier medium,
there is NO PERCEPTIBLE UPPER LIMIT TO THE AMPLITUDE OF EM RADIATION (or
GW radation for that matter). And why is said radiation locked to the
fixed value, c? The floor is yours__________ .