This is a bit complicated, both can be used in this situation. Those is used more specifically when talking to another person and something is far away from you. So you can say "those houses over there". If you're talking about something that is close to you, you can say "these houses here". It is a bit complicated but basically they and those are interchangeable. Those is used in the first sentence to point out what is being talked about. If he just said "they aren't mountains", the other characters may not know what he was talking about. Sorry, it's hard to understand but I hope you know a little more now. (:

The speaker wants to refer to the distance of the mountains (or which mountains), so he started with the demonstrative pronoun "those". Now that the listener knows which mountains, the speaker can start to use the subject pronoun "they" to talk about the specific mountains.