Noise depends on exposure. So you need long exposure for shadows. That would blow out the highlights. With HDR you make several exposures, one for the highlights and one for the shadows and several between.
That's correct, collecting more photons results in lower shot noise. However, in the case of very long exposures the "dark count
" characteristics of the sensor also play a role. With exposures longer than approx. 1 second, the dark noise starts increasing significantly with expose time. The temperature (of the sensor) also plays a big role.
So when the OP talks about 20 seconds for an exposure, his images will suffer. That will require either noise reduction of the image when postprocessing it, or the use of a darkframe subtraction workflow and/or averaging multiple frames. I don't know what technique Dana Neibert uses, his website doesn't support my browser, so if he is not interested in me, then I'm not interested in him either.