It's not that you can't disarm.I could never tolerate this line of thinking if it came up. I’d just shut it down immediately.
The Battlemaster doesn’t mean you can’t try to disarm someone. Obviously.
It's that since the battle master exist, you can't disarm and deal damage in the same action.
90% of the DMs i know and heard of lets every martial character and monster perform all Battlemaster manuevers by trading out the damage.