I do not think your explanation is correct. (Or, more accurately, it should not be correct, based on the card's wording.) There are cards out there that do specify that a card must be from the supply, in order for effect to occur. But Disciple deliberately did not put that limiting writing in . . . it says only "...Action card..." And, given the absence of any limiting words; the commonsense meaning, using English grammar, is that it applies to ANY action card.
I could see an exception (even with the current wording) if you play a card from Black Market, since--by definition--there is only 1 card of that type in the entire game. If you have no additional cards of that type left in the game, obviously, no one can gain extra copies in any way. But there are plenty of "spare" Fugitives in a game, so the clear wording of the card suggests that I should get an extra copy.
If your explanation ends up being correct after all; then the wording on the card should be changed. Maybe adding at the end "...if played card is from the supply, then gain an additional copy" or something like that.
It's either a bug, or it very poor/inaccurate wording on Disciple.