Lots of "just use X" comments but the article is about showing the bare minimum/how easy the core part of routing actually is.
Also, if you have ever used docker or virtual machines with NAT routing (often the default), you've done exactly the same things.
If you have ever enabled the wifi hotspot on an android phone also, you've done pretty much what the article describes on your phone.
All of these use the same Linux kernel features under the hood. In fact there is a good chance this message traversed more than one Linux soft router to get to your screen.
Basically any computer is a router if you're brave enough.
Windows PCs had (have?) that Internet connection sharing feature for a long time. It was really just a checkbox to enable NAT too.
Sometimes I think combining a firewall/router/switch/AP/file server/etc into a device called a "router" really confuses people. Even people who should know better.
Yeah I find it more interesting to see how it's built from scratch, then I can decide if it's worth doing myself or just using X. I think this is a good software principle in general.