That's 29dB gain, and it remains basically universal in AVRs, including this one. You can see the gain measured at 28.7dB in one of the readouts. I'm not actually aware of any AVR that doesn't use 29dB.THX Ultra specification used to be 1V for the input sensitivity to hit 28.28V.
Could the SINAD 100 at 1V reflect that outdated approach?
A slightly odd number - if I had to guess, it's derived from 1Vrms = 100W at @8ohm - the aim being to be able to reach reference level without significantly going above the nominal 1V line level.
There's clearly a fight going on where a lot of people including Amir want to increase source DAC level and reduce amplifier gain for separate amps, but I'm not aware any AVR has made a corresponding internal gain change. For decades they have been set up so that the DACs' 0dBFS = 2Vrms - more than enough to get full power from the 29dB internal amps . They do use the full 2Vrms DAC range, but have analogue volume afterwards before going into the amps and pre-outs. If that analogue volume doesn't attenuate a 0dBFS=2Vrms signal, it will clip the internal amps.
A few amps like high-end Denons will at least allow internal amp disconnection to allow the full 2Vrms clean on the pre-outs.