Unfortunately, that's not true. If a drunk walks out of a bar and gets in his car to sleep it off he's guilty of DUI in most, if not all, states. Even if he never gets in the driver's seat. Even if the keys never left his pocket.
DUI carries with it a BAC requirement, which must be proven. Absent a proof of BAC, no DUI.
Replace "guilty" with "chargeable" and you are correct. However not many prosecutors are going to pursue DUI/DWI charges against a drunk guy who got into his back seat to sleep off his alchohol. Will some? Sure, but most will not.
Now, if he is in the drivers seat, car parked, (some states require keys in ignition), AND BAC can be proven to be in excess of DUI mins, then yes, he can be convicted.
Proof of BAC is established via breath sample or a more accurate method, the blood sample which you must consent to, as in most states DUI/DWI is a misdemeanor, and absent consent, a warrant is required, and most judges will not grant warrants in misdemeanor cases. If you killed someone in a DUI/DWI accident, now it is a felony, and a warrant CAN be issued for your blood.
But with no BAC evidence, no DUI/DWI. Legally, impossible. Suspended license, ride the bus to work? Yes yes. DUI/DWI? No.