That's absolutely the historic Christian understanding. Has been for more than a millennia. They worship God the same way Arians did - they are thinking of the right God, they just have a completely retarded understanding of Him.

Read St. John of Damascus, he calls Islam "the heresy of the Ishmaelites" in the 700s AD - St. Thomas Aquinas takes a similar perspective. Islam, like modern Judaism or Arianism, is a Christian heresy.