Well it's definitely a distinct chain-axe separate from the one held by the Aspiring Champion revealed at LVO. Unless The CSM datasheet is being updated to allow for more than just the Champion to take a chain-axe, It's probably from another kit.
I don't imagine Possessed will have any need for chain axes, so Terminators sounds like as good a guess as any to me. The other possibilities are that the CSM box will double as a Chosen box and will contain multiple chain axes, or the CSM datasheet has been updated has been changed to allow multiple models other than the Champion to take chain axes. Or (long shot) we are getting a separate infantry kit for Chosen or Havocs and this axe is from that kit.
Either way I'm excited. I don't imagine it's berzerkers though.