For an official guide on how to activate WeChat Pay without a China mobile number, what payment sources (primarily debit or credit cards issued by a mainland Chinese bank) are accepted, and what identity documents are accepted (including non-Chinese passports), you can go to WeChat's official Help Center site in English here:
https://help.wechat.com/cgi-bin/newreadtemplate?t=help_cente...
Perhaps the official docs offer a more clear explanation.