The problem, is that the site you quote, knows just enough to be dangerous! People think they know what they are talking about, and they spread enough misinformation to get people to make bad choices. Consider sites like that the Monster Cables of the engineering world. Yes, there are plently of them out there.

Those who promote using 1, 6, and 11 are right, if you were setting up a system with no other interfering signals. But that is not the world we live in. Note that the power will also not be spread evenly across the frequency spectrum for a channel, because of the modulation used. So, if you have several neighbors a medium distance away, and they are using 1, 6, and 11 already, you may find the best signal at 3, 4, 8 or 9. This is because the signal strength you receive from your neighbor may be falling below the noise floor, and allowing for the lowest interference on those channels.

This is but one of the reasons they chose more channels. There are lots of others, and we haven't even entered into discussions of MiMo or other issues. This is not something that is my specialty, but I do know individuals whom it is, and I trust they got this stuff right.

A decent WiFi access point if unconstrained by you telling it to use a channel, will occasionally try various channels, and find those with the best throughput. So unless you really know what you are doing, choosing a specific channel can damage throughput, when another device is nearby using that channel, if your router is then set to use only that channel. I'd recommend that unless you have specific knowledge, and are ready to test the network on a regular basis to make sure the channel assignments are still the most valid, that you let the hardware decide.