This is one of the reasons I quit believing in a pre trib rapture quite some time ago. To think that the Holy Spirit leaves the world, then that those who are left behind get saved in some cases is just not believable.
It doesn't mean that the Holy Spirit leaves the world. That would be impossible because God is omni-present....everywhere present.
What it means is that the one currently holding back the onslaught of evil stops holding it back. "He who now prevents will prevent until he be taken out of the way." That is: He was standing in the way of evil. He steps out of the way of evil.
That is a mystery. But it may well be that the mystery includes a 3rd possibility we can't conceive of yet.