Nah, rewatch the video in the OP. The tackle motion started well inbounds. Both players feet were still inbounds for the entire incident. The fact that they ended up out of bounds has no relevance. If the tackle started over the line I'd agree with you.I don't want Nyhuis drawn and quartered, but I think the damning thing was that the slam occurred when they were already out of bounds. That really does invalidate the 'it was a football act' defence. Once he'd tackled Gray over the line, there was zero football reason to slam him into the turf, whether he realised that Gray had already dropped the footy or not.
I think it's too far to say that he intended to KO Gray, but he did something unnecessary and reckless in the name of making his opponent earn it, and his opponent was hurt badly as a result. In today's AFL you have to cop your whack for that.