as per my knowledge no, user credentials (username and password) are not stored into postgreSQL. The only credentials that are stored there are the ones of technical users used inside postgreSQL itself (see users postgresql and qliksenserepository).
For 'regular' end users, only a reference of their user directory ("domain", if you're working with active directory) and user id ("username", if you're working with active directory) are kept. Once the IdP has verified your identity, will let you proceed to Qlik Sense. Only at this point the IdP will tell Qlik who you are (user directory + user id).