You need to go back and reread it then.
https://en.wikipedia.org/wiki/Phantom_vibration_syndrome
"...Phantom ringing may be experienced while taking a shower, watching television, or using a noisy device. Humans are particularly sensitive to auditory tones between 1,000 and 6,000
hertz, and basic mobile phone ringers often fall within this range...
In the past I thought I experienced the syndrome because I felt vibrating on the back were I put the phone, but I didn't receive any call or text message (I'm talking about the pre smartphone era).
What I didn't mention in the post you quoted is that me, and other people, hear the notification. I guess it is hard for two people in a room to experience the syndrome at the same time.
Last time I was in my bedroom, the tv off, and my wife told me "you just receive a text, I think is your phone" but none of our iPhones had any notification and we both hear the sound. I'm pretty sure it came from my phone because is closer to me.
Maybe it was the sound of an old notification delivered after some minutes.
The Watch receives notifications only when is on my wrist so during the night I can't feel anything. I'm not sure it is a bug, maybe I'm really experiencing the syndrome with the Watch during the day. My point is that there is a bug in iOS when dealing with notifications, so a similar bug may exist on watchOS