There is no better way to train your deadlift grip than deadlifting with that grip.
IMO if you use mixed grip instead of training it you should just not use straps. This will build the callouses up so that you dont tear them every time you do a heavy deadlift on the platform. With a ton of respect I think this is where Rubish has gone wrong.
Using Hook grip I think there is a lot more merit in training grip strength in the beginning because the thumb skin rips off so much easier, or at least for me. But once those callouses are built up I assume I can do more volume with Hook.