Sorry if some think it is a dumb question, but does this kind of training help anything ?
Not saying it dont, but have difficulties seeing any gain as the movements are not exactly the same, at least not when the board/plate is not fixed to the roller ?
It will practice your overall balance and coordination on these specific "toys" maybe, but if not spot on target for hydrofoiling (and you got some overall balance from other boards), does it really matter then ?
It is hard to say whether it does or does not help - as you will never know how well you would have learned without - and vice versa, IMO
Just a thought I have had always, reading about these, or trying them myself