The Greek letter chi is an "x" and the Greek word for Christ is chi and also "X." Xtian has been around since the first century.
You're right. And a handful of people, maybe, know that X is chi and that it is the first letter of the Greek word that translates to Christos or Christ.
Also, ΙΧΘΥΣ, an acrostic of five Greek words, and what's called the Sign of the Fish, stands for "Jesus Christ, God's Son, Savior." But how many people know that?
Meanwhile, we ain't in the first century. And I'll stick by what I said re: IMO, using X for Christ these days is one way to remove Christ from Christian (and Christmas, too). And I still find it offensive.
Some people don't, though. Their choice, including if and when they use it. So be it.