Although Gou Dan couldn't determine the strength of those Secret Eye cultists, Xi Wei saw through them immediately from his Divine Kingdom.
"Demons hatched from demonic roes? No, that's not it… these things are less powerful than Angora's eldest brother, Cecil—and he was only halfway through his demonization back then."
On that note, one shouldn't be too harsh on how Cecil was instakilled by Angora—he simply had a sacred item combining Xi Wei and Aslan's powers, and that thing could wipe out Edward's party immediately too.
Meanwhile, the Secret Eye cultists who were now chasing after Gou Dan does have some demonic scent to them with each of them looking like double images from Xi Wei's perspective, they were much weaker. That was especially evident in how Gou Dan was caught off guard, and yet their combined barrage still didn't blow him up.
"Their combat ability would rank around Level 28 on average then."