Well, that's not "tea or coffee?" as you can have both for the same (very affordable) price
In fact Calligra is included in the Slackware Linux distribution, and in addition Alien Bob provides "unofficial" (but working very well) packages for LibreOffice.
Maybe your surprise comes from the fact that Alien Bob on one hand is a major contributor to Slackware and on the other hand provides SlackBuilds and packages not included in Slackware but intended to be used with it.
As Slackware can't include everything, additional packages and SlackBuilds are provided by volunteers, this being an example of that.
Have a Great Day as well!