I don't think it's as prescriptive as you seem to.
merely collating what he sees are the central 'tenets' of Atheism or Heathenism
Well in the first place, that would not be a manifesto.
In the second place, it's not very accurate as a description. Are those my "central tenets"? I am an atheist, but atheism itself is not one of my "central tenets", any more than my disbelief that there's an elephant in my back yard is one of my "central tenets". I just checked my back yard and there's no elephant, it's not an ideology.