Trying to better understand the issue of T183466, I create a temporary account (Test20180327) on enwiki, and tried Site(code, 'wikipedia').login() (without autocreate) several times on different wikis. The account was created automatically every time.
Then I noticed that the parameter is only respected if the password is not saved in a password file.
Users who have a password file will have an account created for them regardless of the value of autocreate.
This difference in behaviour is unexpected. We either want the accounts to be auto-created by default or we don't, it should not depend on password file.