Although multi-threaded engines are now standard, this model adds complexity and is overkill for most games beginners will make (and many commercial indie titles). There is also no reason not to start with a single threaded renderer and move to multi-threading in a later article. Right now it isn't even past creating a window...
Honestly, if you're going to be so critical you should suggest the tutorial should use Vulkan instead of OpenGL, but wait neither of those support all gaming platforms so you have to write a low level API wrapper for Metal and DirectX12 as well, then write a shader translator... Etc. The gap is large between a toy engine and a commercial engine, but there are a lot of fun games you can make with a toy.
Honestly, if you're going to be so critical you should suggest the tutorial should use Vulkan instead of OpenGL, but wait neither of those support all gaming platforms so you have to write a low level API wrapper for Metal and DirectX12 as well, then write a shader translator... Etc. The gap is large between a toy engine and a commercial engine, but there are a lot of fun games you can make with a toy.