Would you say that the God of Israel, the God of Abraham who was/is worshiped by the Jews is the same God as the God of Chrisitans? While the Jews do not recognize of understand the Trinity, no Chrisitan would deny that the God of Israel is the same God we worship.
Similarly Mohmmed said that the revelation he was brining to the people was that there was only one God, he was the God of Abraham. Now Allah, (God in Arabic) is the same God, but as with the Jews, not understood as Triune, and while the Muslims claim that the One God, the God of Abraham is the God they worship, they do not know him properly. If we deny that Muslims belive in the same God as we do, then we have to deny that Jews, Seventh Day Adventists, Jehovah’s Witnesses, and other non-trinitarian sects don’t simply believe in a mis-representation of the True God, but don’t believe in him at all.
Stating that they believe in the same God, does not imply that everything they believe about God is correctly interpreted by their Religion.