I think I remember trying and failing to use Alipay to make a Baht payment to a merchant in Thailand, with a message that payment in foreign currencies was only available to users registered with a Chinese ID card.

Yes, Wechat and Alipay need to be registered to a Chinese citizen (with Chinese ID) to be able to make any sort of payment to a foreign (non Chinese) company. You can not make payments outside of China, not even in HK which I found out while trying and failing to buy a burger in McD's. I recently got ID checked on Alipay when receiving a payment for over 10,000RMB, they required my passport copy and me to enter the information from said passport manually, not a big deal but a bit of a pain.