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I was just thinking about it and the only situation when this cookie could indeed work would be if their auth is based on client-stored JWT. But this is clearly not the case with Coursera as they have pretty extensive user profiles etc.
Maybe they have some kind of internal identity management solution that is separate from the main web app which is managed separately which is the reason for at least temporary retention even if you delete account.
In any case, it's messy :)