I think they mean that GDScript is the best language for Godot. And at that time of writing it was, they tried several scripting languages(Lua, Python, Squirrel

I think they mean that GDScript is the best language for Godot. And at that time of writing it was, they tried several scripting languages(Lua, Python, Squirrel iirc) and none gave the 'seamless experience' that GDScript. Hell even the C# support at the beginning of Godot 3.0 was wonky. But they listened to the community and added support for adding your own scripting language and Mono support. Which is still a big positive.