The review states the noise is measure with the output volume set to max, which is 15 dB gain. The volume when listening normally would be 20-40 dB lower than that. The noise floor will undoubtedly drop at reduced volumes. So we cannot know if the Onkyo's noise will dominate at the speaker terminals from the data at hand.
The amp's noise is around -90 dB below 1W (1V in 8 ohms) and has 29 dB gain.
1 watt into 8 ohms is 2.83 volts. 1 volt into 8 ohms is 125 milliwatts or 0.125 watts. 90 dB below 1 watt into 8 ohms is about 90 microvolts.
The above seems to be based on analog domain signal processing inside the AVR.
The architecture of your average AVR is that the DSP which does all of the signal processing including volume setting drives the DACs which drive the power amps. Modern DSPs can be negligible sources of noise since they run entirely in the digital domain.
There is usually an analog I/V converter and gain stage between the DACs and the power amps depending on the voltage gain in the power amps and other factors. This analog stage is fairly easy and economical to make in such a way that its contributions to system noise are very low.
One important point is that if the AVR is used with a digital source which is now common practice, the AVR volume control's influence on output noise may be very slight or nil when the digital source is itself muted or replaced with a dummy source.
As mentioned earlier, the (analog) power amp stage can have noise output that is down in the dozens of microvolts range. 90 microvolts from a few paragraphs back is fine. The DAC can thus be the determining factor when DACs with dynamic range < 95 dB are used which is fairly common in low and mid-line AVRs. The up-market AVRs I've looked at use ca. 110 dB parts which is similar to common practice in professional audio gear.