This is a very sloppy argument. The first citation is a work from the 60s which makes the unsubstantiated claim that some kind of practice was actually the survival of a prehistoric fertility cult. Then later the author says any claim made about a prehistoric society must be taken with a grain of salt... and then he makes a sweeping claim.
As far as I know, the modern historical consensus is that Samhain had no link to the dead and that aspect comes entirely from Christianity. People project a lot onto the past based on their assumptions about what seems pagan and what seems Christian, and what they project onto paganism is things that feel non-modern (belief in the supernatural, practices tied to the cycle of the year, things that seem "spooky.")
As far as I know, the modern historical consensus is that Samhain had no link to the dead and that aspect comes entirely from Christianity. People project a lot onto the past based on their assumptions about what seems pagan and what seems Christian, and what they project onto paganism is things that feel non-modern (belief in the supernatural, practices tied to the cycle of the year, things that seem "spooky.")