Definitely true. I also have my doubts that Fabian would've succeeded, either. I feel like Cave Monster is considerably more powerful than anyone even remotely realizes. Those who seem to know the most are (again) only experientially knowledgeable, like the MC and some survivors now (Andrew, Sharon, and barely Laurie, depending on choices). Although, of course, the rest of those all have considerably less understanding than the MC's own limited grasp of her power. IDK how the MC's power is compared to him, but Fabian is definitely clever, so whatever difference in choices he would've made might have also let him get away successfully.
The Cave Monster is definitely a hard task to complete; even with knowledge, I suspect she's an extraordinarily difficult foe. Maybe that's why she's confined to her pool of water. She might have been imprisoned there anciently to prevent her from devastating most of the world. (That's probably already been suggested by others here, I'm sure.)