It's true that "nominal" (in name only) Christians never had true faith, but a spurious faith. You don't need true faith in order to draw back. Those who received the "knowledge" of the truth in Hebrews 10:26 were at a crossroad and those who drew back to perdition DID NOT BELIEVE TO THE SAVING OF THE SOUL and those who believe to the saving of the soul DID NOT DRAW BACK TO PERDITION. Drawing back would be a manifestation of unbelief.
Someone who has received the "knowledge" of the truth and is among genuine believers and is still in the process of considering the truth, but then ultimately decides to draw back to perdition instead of believing to the saving of the soul still drew back. The truth was revealed to such people, yet they still drew back. That is where they drew back from. You can't draw back from the truth if you have not yet acquired the truth and had the chance to consider the truth, but that does not mean you have to fully accept the truth before you draw back.
If I decide to walk across the Brooklyn bridge and I walk right up to the bridge and am on the verge of stepping onto it, but then instead, I turn and walk away from it, does that mean I didn't turn away and depart from the bridge just because I wasn't actually on the bridge? Of course not. It's the same with these Hebrews who draw back to perdition and do not believe to the saving of the soul.