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If a user has an account on one of the stack exchange site, I thought they had an account on it's Meta site.
This doesn't seem the case in the associated users API route for one user (that's all I've checked, because I don't know of others)
Users on meta sites inherit from users on main sites, but not every main site user will have a meta account.
Meta users are spun up "on demand", the trigger generally being the first time a user visits a meta site.