Maybe I’m looking at the wrong project but on GitHub it has been loading the mysqli version of the db library includes/db/connect_db_mysqli.inc since 2014, but only if it can find the mysqli_connect function. If it can’t it falls back to the old mysql module, and will just give the above error if mysql_connect doesn’t work.
I haven’t bothered to download the source that ports is using, but if it’s the same software then I’d wager it’s still an issue with the web app seeing the mysqli driver. The port makefile lists mysqli as a requirement so the version the port is downloading clearly isn’t coded to require the old extension.