I believe it is mainly the West and Southwest parts of Mindano that are predominately Muslim. Most of it is actually Protestant, with some Catholics and a few Muslims. Some years back I spent some time in the area around Davao, as well as going on a cross-island trip from Cagayan de Oro to Davao by bus (with several stops, including one by a Philippine military patrol!). I saw very few identifiable Muslims.
I guess the crowd my wife ran with was pretty well educated (and were beginning to be spread for work all around the world.) Most have a fairly good tho’ not excellent grasp of geography.
Oops - typo. Should be “Mindanao”.
The country is over 90% Catholic.
You’re right. 80% to 85% of Filipinos, or five out of six, are Catholic. Half of the other 15-20% are Moslem, and are concentrated in western and southern Mindanao. My wife is from Misamis Occidental, a province on the edge of the Moslem area, and she comes from a family of Assembly of God pastors, so she belongs to the fraction of the population that is neither Catholic or Moslem.