If you say so. As a former Christian, I was not taught that Adam had any religion except to obey God’s commandment not to eat of the Tree of Knowledge of Good and Evil. He was just a human being in communion with the Creator of the Universe. If that makes him Muslim in your book, I have no problem except that the Quran and Muslim teachings did not come into existence for another five thousand years or so.
I think if you call Adam a Muslim for having certain characteristics of Islam, such as peace with God, that is the same as calling me (an atheist) a Christian because I’m a good person. People have done that despite knowing better. Some people simply wish to build on emotional qualities rather than intellectual facts. So be it.