Well, my justification is that rebel factions on the planet had never tried anything quite so bold as to attack a space station, and the pirates avoid them as it would cause an escalation and possibly a huge search for the hidden base, therefor the space stations have grown somewhat complacent. The station would be surely be equiped with chlorine scrubbers to safeguard from a leaky gargan breathing apparatus and systems to prevent dangers of a major leak, but those systems would have been damaged from the explosion or overwhelmed with the sheer quantity of chlorine.

Beyond that the tanks are supposed to hold tonnes of chlorine that would bring the atmosphere of the station to leathal levels very rapidly, coupled with a suprise attack disrupting normal activity. 

but i can reduce the numbers if they bother you too much, it wouldnt change much because im sure the empire would be very, very, very displeased with the attack regardless of casualties.