And "X" in this case is something like, "I knew eating a cookie wasn't good for me, but I felt like it and so I did it anyway." It's like the need for justification is just missing, at least in their self-reports.

Thanks for this example -- now I can imagine what "never rationalizing" could be like.

I did not realize there is a third option besides "rationalizing" and "always acting rationally", and I couldn't believe in people acting always rationally (at least not without proper training; but then they would remember what it was like before training). But the possibility of "acting irrationally but not inventing excuses for it" seems much more plausible.