"Amazing" that you think this ego subtext totally invalidates the character of Atreyu the Warrior and Fantasia as a whole. It doesn't.

Atreyu is indeed a warrior out to save the land of Fantasia. Why else does he push on when he could very well have sunk into the Swamps if not for the arrival of Falkor?

He's setting the example for Bastian to follow, which Bastian does. He's the conduit through which Bastian connects to Fantasia, the place he must help save by giving the Empress a new name. His wounded ego is totally allegorical to the book, but doesn't invalidate it. Bastian needed help healing his ego; Fantasia needed a child to help fight the Nothing. They were in the right place at the right time.

Atreyu seeing Bastian in the mirror is his simply seeing the other side of the conduit: that of the human child Atreyu needs to get in touch with to save Fantasia (though he didn't know it yet). And Bastian sees the warrior he wants to be to heal his ego. The two points of view are both interconnected and the Magic Mirror Gate is the nexus. But to haughtily invalidate the entire land of Fantasia and Atreyu and chalk it all up to a singular allegory with "Amazing, isn't it?" I don't think so. That's not what Michael Ende wrote and that's not what the screenwriters wrote.

Remember what Engywook said about most men running away "scrrrrreaming!" from the Mirror Gate when "confronted with their true selves"?

Atreyu didn't do that. The "broken, insecure, little" Bastian did in the real world (but he didn't scream--he was simply freaked out that a fictional character in a book could see him because "This is going too far."). Atreyu faced his fear and continued on to the Southern Oracle. Therefore, Atreyu really was a truly brave warrior. And in the end, Bastian followed his example and faced his fears, saving Fantasia and curing his ego in the process.

As they say, there are two sides to every story (this Neverending Story). You think there is only one. I'm showing you the other side. As you said, "the truth shall set you free."