I thought the author in the Scott R interview posted here said he claimed to become angry and panicked when Bella and Cece supposedly woke up from his first attempt to murder them. That gave me the impression that he isn't admitting in the book to picking out the site beforehand.
So I'm even more confused why he would admit to planning their murders and not that he had pre-picked the disposal site.
I could be wrong since I haven't read the book.
ETA, from interview transcript:
46:57 he said that they woke up I think it threw him into a real tailspin he he became very upset and very angry that they got back up he decided last minute to I think to take them out to the the oil batteries and I think when he grabbed the gas can it was to burn the bodies
Edited to also add: IMO he made up the story about murdering Bella and Cella the first time to explain why he supposedly panicked and took them to the oil batteries last minute. So yet another stupid CW lie because he must be aware that people already know he had prearranged to be there alone. JMO