Continuity mistake: When Charlie Brown is talking to Lucy at her Psychiatrist booth, the sign says "The Doctor is Real In" or "The Doctor is In" depending on the viewing angle.
Continuity mistake: When Linus gives his biblical recital on the true meaning of Christmas, there is a United States flag on stage at the top of the stairs (viewer's right). When the shot changes to a side view of Linus, the flag is not there. (00:20:45 - 00:21:30)
Continuity mistake: The planks that hold up the sign of Lucy's psychiatric booth are red, but when Lucy suggests that Charlie Brown has hypengyophobia and it cuts to Charlie Brown, the left plank is orange. (00:05:22)
Other mistake: Throughout the scene where Lucy is at her psychiatric booth, her body clips through it in many shots. (00:04:36 - 00:05:26)
Continuity mistake: When Lucy talks to Charlie Brown at her psychiatric booth, there is a money jar on the counter. After Charlie Brown screams, "That's it!" and Lucy falls over, the money jar is gone. (00:05:53)
Answer: He never asked about the "true" meaning of Christmas, only that he was depressed about it. He didn't have the Xmas spirit. The feelings of love and compassion. He was talking about Santa Claus and gift-giving, but Linus told him what Christmas is all about.