I'm not sure I'd agree with the comparison to the FM transmitter. That TV DAC claims to have a SNR of 90dB, which is well above the ~70 of FM radio, and comparable to "CD quality" (modulo marketing BS).
That being said, the benefits of 24 bits over 16 (which I'm not convinced of in any case) would certainly not survive that setup.
I was in the cable industry for over 20 years and am well acquainted with NTSC ... I suspect the audio quality at 6 MHz to be far better than an FM signal which is nominally 100KHz wide. The FM system is more resistant to amplitude changes (due to distance, etc) but can't encode as much data as the VSB-AM signal used by NTSC.
There are a few caveats and I haven't looked at the transmitter/receiver pair he purchased. If it's simply 6MHz of bandwidth transmitted for some specified distance, it will probably work. SPDIF is a digital signal and while it runs at 6Mbps, there are significant harmonics that can be passed through the fiber. Since we can assume the first harmonic will pass through the link, you can expect a signal which has more sinusoid pulses coming back out of the receiver. With a PLL to recover the clock and a good data recovery circuit added to the receiver, I'm guessing he could easily push this link to 50 to 100 feet.
As an aside, the NTSC signal is actually sending far more information as a whole and while it uses an AGC to overcome amplitude changes during transmission (the sync pulse is adjusted to be -0.3V), it also uses a comb filter to recover chroma information and the luminense is AM modulated above at the pixel clock rate. Your eyes are actually pretty low resolution compared to your ears and there are lots of ways to fool the brain's image processing (e.g. interlace makes a 30FPS system look more like 60FPS).
That being said, the benefits of 24 bits over 16 (which I'm not convinced of in any case) would certainly not survive that setup.