China as a country has well over 51% of the BTC hashrate.. They could at any point and time decide to run it into the ground or double spend on it..
WOuld kill the golden goose though and I suspect that is why it's not been done.
China is ... a country ... not single person. How you imagine "double spend on it" "at any point" ?
I can tell you even more frightening news. EARTH (our planet) has 100 % of bitcoin hash rate. It can double spend any time... be carrefour.