I have been contesting the use of the word 'tomb' in the classifications of all the types for years. Although certain examples certianly ended their usage as tombs, they almost certainly had a wider function.

They're temples and just as today's churches have burials within so did they.

Oh, and bones and cremated remains were found in Newgrange during excavation, so part of his statement is false. It was used for 'burial', but it was probably not its primary function.