Yes, it's true.
The "X" in Xmas is not defamatory in any way -- far from it.
This "X" has origins that go back for centuries. And there is a reason why it has been used -- it's no mere accident or quick abreviation.A source at The Vatican tells us that the "X" in X-Mas actually comes from two letters from the Greek alphabet, Chi (pronounced Kai) and Rho.
These two letters placed on top of one another symbolized the first two letters of the Greek work for the Messiah, the Christ. So, an abbreviation of the word Christmas is X-Mas (although it has lost that meaning to modern audiences). As you can see, our source has added a diagram that shows you what he means: