I think nobody replied to your prior post, because nobody here has any idea!! Would be a good question for Pat Foster (AMC historian). Could be it was just something that floated up in a marketing guy's alphabet soup during his lunch break.
Along the lines of the OP's question, I've always been interested in how Suzuki also picked up and used the "SX4" nomenclature, minus the slash between the x and 4.
When AMC was purchased by Chrysler, did all the corporate logos and names go up for grabs? I assumed that NOT to be the case as Chrysler released the Dodge Spirit in the 90's and now the Hornet SUV. I don't know trademark law, but the omission of the slash doesn't seem enough to differentiate one "SX/4" from the other "SX4".