Yes, some plugins are automatically enabled based on their *.uplugin definitions. If you go to Engine/Plugins/Runtime/ProceduralMeshComponent/ProceduralMeshComponent.uplugin, you can see it has the “EnabledByDefault” field set to true.
Yeah the documentation can be lacking in places for sure. Luckily we have the full source at our disposal, so we can also find answers by doing some digging. It just depends on how much you want it and whether it’s worth it. I personally wouldn’t worry about it too much since your project is building fine.