Kas'im struggles against a wild rancor while no-name fodder lost tribe sith one-shot nightsister amped ones. He's a trash tier combatant who developed delusions of grandeur because he never actually fought a real battle in his life and could only compare himself to sith trainees.
Bane thinks Kas'im is the greatest of all time in the same way a middle school student thinks his math teacher is comparable to Terence Tao because he can do algebra really quickly.