That statement shows some absolutely fundamental ignorance of Code Division Multiple Access/Collision Avoidance systems and the way Wifi works (Hint: this is a field I’ve spent several decades working with, long before Wifi was a “thing”)
You CAN operate 2 different wifi networks on the same channel physically side by side if you wish. The effect will be about a halving of theoretical speed
The idea of using “non overlapping channels” is simply a rule of thumb to try and achieve best unencumbered throughput - and in fact the best set of channels actually varies depending on the modulation methods used (they actually always overlap, see further down)
- 3 non-overlapping assumes 802.11b
- If you use 802.11g or 802.11n 20MHz and have 13 channels available then you can achieve 4 non-overlapping channels.
- 802.11n 40MHz only gives 2 non-overlapping channels
- 802.11n 80MHz wipes out the entire 2.4GHz band (nobody uses this one)
There’s a reasonable amount of explanaton about this on Wikipedia: List of WLAN channels - Wikipedia
What that means is that if someone’s operating on channel 7 that doesn’t “lockout” adjacent channels:
It simply slows channel 11 down a little and behaves more or less the same as if it was operating on channel 6 if you try to run something else nearby (ie: you’ll start seeing major slowdowns, particularly if your station(computer) is equidistant (attentuation-wise) from the Access Points
In terms of “co channel interference”, if the signal strength of the adjacent access points is more than ~45dB below your desired access point, for all practical purposes “it doesn’t exist”
The fun part is that whilst we talk about overlapping channels as if 802.11b is still in use, the last 802.11b-only chipsets were made more than 17 years ago - and there are such nasty side effects of using 802.11b that the very first thing I advise is DISABLING IT (if any station using an access point uses 802.11b then ALL stations must use 802.11b and they ALL run at the speed of the slowest 802.11b device).
802.11g has been out of production for more than a decade too.
After assessing the prevalance of 802.11b stations in various large-scale academic networks (hundreds to thousands of APs) I disabled that back in 2015 as it hadn’t been seen for several years
More recently I’ve been disabling 802.11g (and 802.11a - its 5GHz brother) - in the main academic network I administer (1500 access points) there were a grand total of 3 802.11g stations - two of which were ancient USB sticks and the other was a 16 year old laptop computer which was plugged into ethernet anyway
The images below give a better representation of the spectrum shoulders of 802.11g/n channelling. OFDM gives much flatter and sharper shoulder cutoffs until you get down to about -30dB where they start spreading out into sideband lobes - as you can see, a zigbee channel is going to see interference from the sideband lobes even when sited “between” the “non overlapping channels” - which in reality aren’t non-overlapping, just minimally slowing each other down.
This is why I recommend that it really does pay to find out which channels are operating around you, then channel your zigbee (and your own wifi) accordingly. Your network will run much more happily
Of course for best wifi results, you’re better off using 5GHz wherever possible.