I think a lot of the "stronger" commentary comes from disc brakes generally providing more braking for a given amount of hand effort compared to rim brakes.
Getting full braking on my road bike - with old, single-pivot Saccon rim brakes - takes literally a white-knuckle effort.
On my nice MTB, I can get all the braking I'd ever want from a moderate, 2-finger effort.
I'd say that calling disc brakes stronger - for the amount of hand effort that feels comfortable and natural to apply - is a quite fitting description for many bikes.