Let me ask you this.
If your character gets stunned, generates a ton of rage while being stunlocked, then comes out of stun. Whats the first thing thats gonna happen? His auto attack goes off. And he can then proceed to use WW or MS or BT.
Literally the same concept here. Slam cast time is acting as a stun (except he has to make sure he doesnt attack first so that slam doesnt interrupt the weapon swing timer) and when slam is done casting his auto attack casts right after because he has not attacked yet