Well, that is a bit too more complicated in fact
1) he was born in the Czech Republic to Czech parents, but to be fair, he lived in Austria since his childhood. There he learned to ski. As he said he has not so many relations to czechia now anymore, he barely remember last time he was there and he basically said he has no friends anymore there at all for example.
2) The Austrian Federation offered him before this season better conditions, which he obviously took advantage of. Possibilities and condittions he would never had the chance to receive in CZE.
https://www.idnes.cz/sport/lyzovani-snowboarding/matej-svancer-akrobaticke-lyzovani-prestup-cesko-rakousko-big-air.A210330_184239_lyzovani_ten