“Not your keys, not your coins”

The phrase compresses Bitcoin’s custody reality: whoever holds the private keys controls the coins, and an exchange balance is therefore an IOU from a company — one that evaporates in a hack or bankruptcy, as Mt. Gox, Celsius and FTX each proved at scale.

It is not anti-exchange dogma; on-ramps are how most people buy. It is a reminder of what changes the moment you withdraw: counterparty risk drops to zero.