Sorry, it gets a bit more complicated. It could in fact be that false prophets earlier in their life believed the Gospel for a moment and were eternally saved.
It is also true that those who listen to a false prophet, even though the false prophet has a false message, may be saved.
It may be that the false prophet presents a Bible verse such as John 6:47 or First Corinthians 15:1-4, and in that moment, hearers are saved, so the reality is a bit more complicated than simply saying these are damned people preaching to damned people.
The Bible is very clear that some people who are not believers preach the gospel for money and get people saved.
What we are saying is that false prophets are preaching a false gospel and if that gospel is believed, it does not have the power to save people.