Placing a net tie underneath another component without the DRC error?
You can add another Component Clearance rule under Placement with a custom query such as ComponentType = 'Net Tie', HasFootprint('nettie'), DesignItemID ='nettie' etc.

Set the vertical clearance for that rule to 0 and horizontal clearance to whatever you need.

Set the priority of this rule above the other component rules, so Altium wouldn't complain about it having vertical clearance issues.