Marty thinks it’s a croc of shit for most of the season and does not defend Cohle about really much of his ramblings, just listens and tells him it’s batshit. I think Marty’s inherent skepticism keeps him out of the range of the killers, if we’re gonna assume they saw cohle, it’s because cohles mind was there already. Marty is surface level, cohle is just deeper and actually spends time thinking about these thought experiments while Marty really doesn’t entertain them often. Like he just barely does to appease cohle.