Ah yet again the assumption that Neanderthals were not civilised (comparing them to our ancestors that is). Of course they were a compatible race to homosapien sapiens otherwise no interbreeding would have taken place. All humanoid beings have an inbuilt belief in the afterlife and spirit. I am no scientist but intuition tells me that all humanoids are alike when it comes to life, death and spiritually.
Even animals are sentient beings that have an understanding of death and spirit. I would venture to say that Neanderthals would have also been intuitive.
https://humanorigins.si.edu/evidence/genetics/ancient-dna-and-neanderthals/dna-genotypes-and-phenotypes Check this out, evidence of Neanderthals had O blood group.O negative being the psychic blood line. Most royalties on the planet are O negative.