I can't say for sure not knowing the person other from his videos. But he could be fitting those tires to a car or kart or any other not road legal vehicle he uses to have fun on his own land (in the field). So nothing is preventing you from doing this if you don't use them on public roads or sell them afterwards.

This was actually filmed in Finland. And the person doing this is well aware of the risks of regrooving/re-treading and is NOT, I repeat NOT doing this to sell them afterwards as new tires. He's just demonstating the method at which even car tires were regrooved back in the days when safety was not a concern and it…