The great catholic historian Hillaire Belloc considered Islam to be a heresy in his book the Great Heresies. Muslims do not believe in the Trinity, therefore Allah is not God. Simple as that.
You may be quite right in you conclusion (!) (!)...
... and yet it cannot be "simple as that." If it were "simple as that," we could also say:
"Jews do not believe in the Trinity, therefore YHWH is not God. Simple as that."
"Christians believe in the Trinity, therefore they do not worship the God of the Torah (Old Testament) (Hebrew Scriptures)."
Keep in mind that neither Jews not Muslims are Trinitarians.