They are not, but with sound signals, recorded by microphones, reality looks like this:
 The phase argument is with microphone recordings in so far neglectable as the RMS value is usually computed to produce a very low-frequency envelope. The standard frequency argument of the Nyquist RMS function is e.g. 100Hz and to produce a 180 degree phase-shift at 100Hz the stereo channels must be recorded with two microphones that are placed a distance of minimum 1.72 meters or 5.5 feet apart:
344 meters per second = average sound velocity in plain air
100 Hertz = 100 periods per second
period / 2 = 180 degree phase shift
344 / 100 / 2 = 1.72 meters = 5.5 feet
Okay, this is not an unrealistic microphone distance in symphony orchestra recordings. But people wo do such recordings know that such a microphone arrangement produces “swampy” bass signals and therefore frequencies below approx. 300Hz are usually either recorded with an extra mono microphone or alternatively the bass signals of the microphones are mixed to mono in the microphone recording mixer anyway to “ground” the bass fundament.
 In a 100Hz envelope the phase relation of frequencies above 200Hz in the original signal have no significant influence to the summed RMS values of both channels, where a microphone recording with heavy phase problems sounds so bad that everybody would instantly throw it away instead of trying to compute an RMS envelope out of it.
 In practice the envelopes are computed with maximum 10Hz (usually even lower) to prevent interpolations with the audio bass signals, what means that the microphones must be minimum 8.5 meters or 28 feet apart to produce a 180 degree phase difference and frequencies in the audible range have no significant influence to the summed RMS values of the stereo channels anymore.
This means that the easiest way to compute a 10Hz RMS envelope from a stereo microphone recording signal is just simply SUM the RMS values of both channels and then divide the result by 2, where (mult 0.5 x) has the same result as (/ x 2):
(mult 0.5 (sum (rms (aref s 0) 10)
(rms (aref s 1) 10)))
I do not want to say that this is the mathematically over-correct method to compute the mono RMS envelope of a stereo signal, but in an 10Hz envelope phase differences above 20Hz do not really matter in practice. There may be edge-cases like bass compressors, but the main perception range of the human hearing is approx 100Hz to 10kHz, and a 100Hz stereo phase difference does not matter in a 10Hz envelope.
Yes, two exactly 180 degree phase-shifted mono signals produce silence, but I never have found such signals in natural sounds, that usually contain a mix of many frequencies with many different phase relations and more likely behave like equally distributed noise. Also a simple delay (like produced by far away placed microphones) does not produce a constant phase-shift over the complete frequency range from 20Hz to 20kHz, instead it produces a is frequency dependent pase-shift like a comb-filter.
Everybody who writes a filter that produces a constant 180-degree phase-shift with all frequencies from 20Hz up to 20kHz wins a big cake!