Ok, this may seem like a stupid question for long-time players, but as a noob reading the rules as they're written, here is my question: so the Deep Strike Special Rule says you cannot Assault the turn you Deep Strike, however, the Slow & Purposeful Special Rule says you can Assault after firing a Heavy weapon. Does the Deep Strike Special Rule overrule the Slow & Purposeful Special Rule, or can Obliterators Deep Strike in, fire a Heavy weapon, and because they're Slow & Purposeful then Assault?

Now, I understand that most likely the part in the Slow & Purposeful Special Rule was put there so Units with it wouldn't have to slowly make their way across the battlefield and stand still for a turn to fire their weapon, then can't Assault, but as it reads to me, it's saying anytime a Slow & Purposeful Unit fires a Heavy weapon, it can Assault. If that is not the case, what in the Rulebook explains that?