The PD applies to everyone the Federation encounters, pre- or post-warp.

The PD prevented the Federation from intervening in Bajoran internal affairs (DS9: “The Circle”). It prevented the Federation from intervening in the Klingon Civil War (TNG: “Redemption”). It prevented Picard from interfering with the Kaelon tradition of elder euthanasia, despite Lwaxana’s entreaties (TNG: “Half a Life”) and with the Ligonian’s racist culture (TNG: “Code of Honor”). Kirk assured the Vians that even though they were more advanced than the Federation, the PD applied to them as well (TOS: “The Empath”).