I don’t fully understand the limitations on picking an engine. Am I allowed to use Engines like Unity, Godot, Unreal, etc. 

They mean that you can play the game without needing to download another software with it 

you can use any engine, as long as you dont have to download an external thing, like the roblox client for example
using unity/godot/unreal is just fine

Oh ok that makes sense. Thanks for the clarification.