That's a good point, I'd assumed since they're using the MIT License that you only have to reference the license with the source code, but I see they state otherwise in the link provided by Dark Dimension. I'm going to go with the following recommendation there and place a link in all my itch.io game descriptions that use Godot (and also in credits if the game has in-game credits).
"The Godot Engine developers consider that a link to this page (godotengine.org/license) in your game documentation or credits would be an acceptable way to satisfy the license terms."