3 Body Problem - Do you believe in Santa Claus? 

I believe he means that the aliens (santa), in the context of her personal safety, don't matter. The dangerous people who believe in him *are* real, however, and their presents (violence, threats, etc) also exist.

Even if Santa is not real, people do get gifts on Christmas. Even if the San Ti Aliens aren’t real, people do get fucking murdered for being involved with the San Ti cult. The ultimate reason why she could get murdered isn’t important, it’s the fact that she could. This is just one of the things this could mean.

"you don't need to believe in Santa to believe people give gifts on Christmas?" The quote doesn't make any sense or does it? People give gifts on Christmas BECAUSE they don't believe in Santa?!

It means that regardless of whether or not the aliens are real, there are actual people doing dangerous things that need to be dealt with

She asked if he believed they were in contact with aliens. he said, one doesn't have to believe in Santa clause (aliens) to believe people give gifts on Christmas. Hes saying that he doesn't necessarily have to believe that they've contacted aliens to know that they're doing something shady. Maybe its not as grand as aliens.