> Christians believe that for folks who don't know Jesus, nihilism is essentially true

Some Christians believe this. Some Christians don't. And some Christians believe something which technically agrees with this, but doesn't actually hold that knowing Jesus by name or having any contact with formal Christianity is essential, which makes it functionally more like the reverse ("people for whom nihilism is true don't know Christ").

Christians, even by the narrow creedal definition, believe lots of different things.