That makes sense.
You prime the screed/concrete for two principal reasons.
1. to divorce different chemical substances e.g anhydrite form cement or gypsum finish from render. This one is irrelevent in this instance as it is cement on cement.
2. to reduce suction from the adhesive into the substrate. If the substrate is too dry it will suck the life out of the adhesive and cause it to "set" prematureley (false set) and subsequently fail. In the case of a green screed it will still be quite damp and so will not apply such a high level of suction to the adhesive. I see this effect in screeds direct onto concrete and concrete direct to hardcore where the application will always involve soaking the substrate with water to stop is "sucking" all the moisture from the topping.
