But surely there there has only been existence for 15 billion years.

As time and existence are inextricably bound then it doesn't mean anything to say, "There has never been an instant in time when there was non-existence."

Other than the last 15 billion years there was a state of non-existence. Not that we can really consider it a state.

Anyway, in terms of the first quote, it is essentially talking about mind and not just the existence of matter, energy etc.

Blacknad.