What is unique about the worship of Molech that distinguishes it from other avodah zara enough that it deserves discussion and specific prohibition in the Torah? We see it specifically discuessed here.
The Ramban on Leviticus 20:5 addresses the question. He says you have to answer differently based on the machloket in the gemara on what Molech was. But by simple understanding that it was an idol, the prohibition comes to teach us that you can't serve another idol (not Molech) by passing your child through fire. Even though that was not the way that particular idol was served.