I think calling it a demon is their way of understanding it though. In spiritual terms, things like autism, epilepsy, even deafness and dumbness are considered demons. But, as you say, you are an atheist so this doesn't really mean much to you.

It is how people who don't really understand a phenomenon rationalize it for themselves. Now, there's nothing wrong with that, but they allowed it to influence their decision making in terms of their business. Those are typically not the sort of people to be employed by in any field, so if I were you, maybe cut your losses and seek employment elsewhere I think.