Possible brake failure?

The above highlighted words sound self-contradictory to me. Sliding normally means traction failure on a slippery surface, not brake engagement failure. Sliding generally indicates full brake engagement, but possibly with an ABS failure to prevent brake lock-up.

Could you clarify?

From piecing together past descriptions and federal passenger car braking regulations, I do wonder if actual standing on the brake pedal will override the power assist and electronic braking control layers, and put enough manual force into the hydraulic system to actually lock the brakes. This could be a result of federal requirements that a certain amount of unassisted manual braking still be available to override a failure of the power assist system, if the pedal is pressed hard enough.

What is the condition of your tires, e.g. type and remaining tread depth? What were the road surface conditions, e.g. dry or wet or icy? Rough asphalt or polished concrete or metal plates or grate?