China outlawed crypto mining early last year, resulting in a two-thirds drop in the hashrate within a month. Even then, the order of magnitude was wrong for it to be "the best way of exporting coal," which by the way China is a large net importer of; https://ccaf.io/cbeci/index estimates the whole Bitcoin network at 14 GW, but China's electrical production averages 850 GW, and overall energy consumption is some 4500 GW, 2500 of which is still coal.

So, at best, China could only have been exporting about 1% of its coal consumption in the form of Bitcoin (though in fact much of its Bitcoin mining was renewables-powered), and now it has mostly stopped doing even that.