There is some confusion about the passsync user because our language is very misleading in the command-line help:

--passsync=PASSSYNC Password for the Windows PassSync user

and not much better in the man page:

--passsync=PASSSYNC_PWD
Password for the Windows PassSync user. Required when using
--winsync. This does not mean you have to use the PassSync service

The passsync user is a special bind user we create for the Windows PassSync service to use to change passwords in IPA. It skips over policy checking because it is assumed that AD has already done this, and by the time we get the password it is too late to reject it. The password is also created as non-expired.