Reason why I personally didn’t find this useful, is due to the fact that there are already a load of topics that discuss metatables. That also includes pretty much everything in this tutorial.
Tons of Posts
All you need to know about Metatables and Metamethods
OLD Metatables, Metamethods, ModuleScripts and OOP
Metatables, told by a guy obsessed with Lua
Additionally this is discussed in the programming in Lua ebook
https://www.lua.org/pil/13.html