I remember they mentioned something about it back in the day, but I won't have the sources to quote. However, you can think of the Primal Menace more as a quest mission/puzzle than a boss. It does not drop any loot and was not meant to be killed multiple times by the same character other than the 11 times you must do it to complete the quest (considering, of course, failed attempts). That's why it also does not drop any loot.
Now, this immediately reminds me of The Mega Magmaoid, which is pretty much the same thing, and the difference between them is... Almost none. If you ask me, the Magmaoid should also not be in the Bosstiary, but if anything I'm glad it's not the opposite. One key aspect of The Primal Menace is that it can be killed in several levels of difficulty, and farming it at Hazard 1 would not be faithful to its real challenge, which is of course defeating it at higher Hazard levels. We would end up having players with 60 Primal Menace kills but all of them at Hazard 1, and would be odd saying that the player has "Mastery" even though he only did the easy part. The Mega Magmaoid is always the same, at least.