Stainless steel is harder than brass (Copper/Zinc alloy).
When wet tumbling, water likely acts as buffer to slow down SS pin movement to gently clean brass surface but when dry tumbling, vibratory action could act as "impact hammer" to contact brass surface with added weight of brass in the bowl. Wet tumbling usually involves slow rotating drum action.
And likely why dry tumbling uses lightweight media that are softer than brass (crushed walnut shell and corn cob) as vibratory action with combined weight of brass in the bowl, the media acts like padding between brass cases to clean/polish brass surfaces.
For me, I would not dry tumble brass with SS pins.