@Barmar then the Bible would not be revealed truth, would it? The reductive argument doesn't work if it doesn't start with the YEC premises, including the premise that the Bible is revealed truth.

You can't prove something by assuming the proposition, that's circular reasoning.

@Barmar I'm not trying to prove YEC is true. I'm just responding to the reductio ad absurdum in the OP, and showing that the logical reduction doesn't work.

@Barmar all reasoning is circular since you've got to start with assumptions... YECs assume that God is honest and that the Bible is his word. Whether you think those assumptions are true or not is a different question from whether the logic proceeding from them is sound.

+1: similar to the response to Can an omnipotent God create a stone so heavy that God cannot lift it? to which the answer is presumably Yes, God could create such a stone, but in reality is unlikely to do so since God does not exist to entertain philosophers.

"In order to successfully argue a reductio ad absurdum it is necessary to accept, for sake of argument, the premises of one's opponent." This is what many people arguing from the Problem of Evil fail to do. Up front they verbally assent to the idea of an omniscient and omnipotent God, but then quietly slip a non-omniscient or non-omnipotent entity into God's place in the argument.

@EvilSnack well-said; I have seen this happen many times.

@EvilSnack "Up front they verbally assent to the idea of an omniscient and omnipotent God..." That one of the premises of Last Thursdayism. It could be argued that YECs are saying "but not omnipotent enough to create all those consistent false memories".

No that's not it at all. The YEC position is arguing for an age of the universe based upon a YEC reading of the Biblical text. It's not a question of could God have created false memories, it's simply a matter of what did He do, if the premise that the Bible is revealed truth is accepted. Nobody has to accept this premise, but without accepting this premise for sake of argument, one cannot do a reductio ad absurdum.