Though both are Dark Stars, the Dark Star coming from the Gate of No Boundary in Gemez Fortress is not even on the same level as the Dark Star known to the Ancient Man Snake.
The Dark Stars from Gemez Fortress are transmitted over due to the limitations of the Gate of No Boundary, usually the weaker individuals with unique abilities but with power levels likely only at the Master of Stars level.
However, the Dark Star with the Ancient Man Snake is actually a relatively small God Star, who has been sleeping here for an unknown number of years. It's uncertain how much of its power remains, but just from its size alone, the combined size of the twelve Dark Stars from Gemez Fortress wouldn't match this Dark Star.
But even if they couldn't match it, with the strength of a Dark Star, it can still single-handedly take on the Master of Stars, especially when possessing special skills. With a slight misstep, the Monarch could get hurt.
