The Loom in a flat panel can be easily cast-off but you need to determine the type of cast-off required. This is the Non-Stretchy Version of casting off. This type of bind-off has no stretch and is tight. So if the other side, the starting side, is more flexible, this will not be a great solution.
Truth be told, I’m unsure why this is even suggested but what do I know. I’ve never done a project so far where a non-stretchy bind off works well.