How so. In popular belief the inverted cross is more known for its use in anti-Christian symbolism than as a sign of humility. That said, even for people who know both meanings, it is entirely reasonable to expect that when an inverted cross is used in fiction (and by a person dressed in all black for good measure), it is more likely for its more commonly known and used anti-Christ meaning to be intended, as that is usually the case.
Instead of implying that people are ignorant, I'd sooner ask "isn't it a bit dumb that Lab Zero didn't think a bit more about it before using a controversial symbol?". And I'd answer that no, they surely thought about it enough and knew what they were doing - but in result, neither the dev nor the fans have reasons to blame/shame people for the fact that Parasoul gets associated with Satanism.