Malz's ult interacting with Kennen's

So I tried to ult him during his ult, and it went off so he was targetable and less than 1 second later he was still moving. I tried it again, same thing. How and why? If he was doing lightning rush during his ult and before me ulting, then he shouldn't be at all targetable and my ult shouldn't be wasted. Refund something or just don't make it go off, please. If my ult actually worked, then he was suppressed, and he couldn't've activated lightning rush to move. Or is he an exception?