Earlier quoted context omitted.
How is that enforced? What is the technical basis that enforces read-only access using user/password auth? Especially since that user/password auth is used by an end user to do "write"-type actions?
Read-only access is not possible. By handing over the credentials you are handing over write access. You are correct.
Citi and Capital One have OAuth flows that Plaid supports, too, which tends to make me angrier at the banks than Plaid; the need for this stuff has been clear for a decade now, but only a few have added OAuth or similar.