Well, there is only one Supreme Being.
Anyone who recognizes that fact, and who intends to worship that one Supreme Being, is a monotheist and is in fact worshiping that one Supreme Being.
Muslims do that. They also get absolutely everything about God dead wrong, and I am certain that Satan himself has a great deal to do with Islam, including posing as God at times.
What did they say before Vatican II about Islam?
Absolutely Arthur.
Your post is timely - this was my Scripture verse of the day from Biblegateway:
"let it be known to all of you and to all the people of Israel that by the name of Jesus Christ of Nazareth, whom you crucified, whom God raised from the deadby him this man is standing before you well. And there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved.
Acts 4:10,12
No one worships the Supreme Being if they knowingly and vincibly reject His Son.