So? Large majority code running in any distro was developed by third parties. This means nothing. It's a part of Slackware.

It was announced in Tue Sep 16 11:20:30 PDT 2003 that it's a part of 'extra' repository. So, not installed by default, but it's there.

Announcent on Fri Nov 21 13:51:57 CST 2008 says it was moved from extra/ to ap/, meaning installed by default on all except the most stripped down installs.

Also, there's nothing special in using third party tools as package managers, they all use slackwares pkg tools under the hood. Think (apt<->slackpkg, dpkg -i <-> installpkg, dpkg -r <-> removepkg, etc etc etc).

slackpkg ships as part of the default slackware installation. Are you thinking of sbopkg which is used to build packages from slackbuilds.org?