I'm just guessing here but they might be hedging against future trade restrictions set by China against US companies like Tesla. If another trade war was to happen, people in China might be able to buy a Tesla using Bitcoin vs having to buy a NIO.
I think there are a couple issues with this idea: firstly, they'd be accepting the Bitcoin, so they wouldn't need any on hand to process the purchases. Secondly, if China wants to prevent or restrict the sale of a physical good, especially one as large as a car, it's far easier to stop it from being imported (or manufactured) than to stop people from paying for it.
Cars are regulated beyond just the point of sale. I believe it would be rather difficult to get said car registered (presuming there's a Chinese equivalent to vehicle registration), if you could even arrange receipt in the first place ;)
How do you imagine that pans out for the Chinese buyer?