The author glosses over the fact that you can upgrade to any new macOS version for free for around 7 years after your Mac was sold as new.
And even then there are ways to prolong the support window with projects like the OpenCore Legacy Patcher https://dortania.github.io/OpenCore-Legacy-Patcher/
So you buy a brand new Mac, you get to upgrade to the latest version and then even if you don't use OpenCore Legacy Patcher you're still getting at least some of the security updates from the latest version of macOS.
I realize that the author paints the picture of that later reduced level of support as being a bad thing, but they don't actually go as far as supporting their arguments with any sort of evidence beyond "there are bugs!"
Out of the box Apple is giving you ~10 years of at least partial updates and you can prolong it further using third party solutions like the OpenCore Legacy Patcher.
For example, you can use Monterey on the Mac Mini you bought in 2009, and the only way to go back to an older model is to start talking about Macs that are physically incapable of running modern macOS because they have physically incompatible 32-bit processors.