Pretty decently executed ass-pull.
I'm both not surprised by the turn of events but intrigued by the ramifications thereafter. Going full Kaiju will probably be the tipping point in this battle but it will also likely mark Kafka as a danger to society. Might even get locked up for it.
There's also the possibility he can't turn it off after one minute and loses complete control and he's to have it beaten out of him/force his human persona back to the forefront, all of these I'm sure will unfold at some point in the story.