>It says they worship THE one merciful God, but the one merciful God is Holy Trinity of God The Father, Jesus Christ the Son, and the Holy Spirit
Yes, it's true, that's a true fact about the one true God, and they believe in errors about true God. But this doesn't mean that they believe in different god, merely that they, while believing in true God, also believe some things about Him that aren't true.
To use an example: suppose I believe that George Washington had a Turkish father. This is of course incorrect, because he didn't have a Turkish father - but it wouldn't mean that I don't know that the historical George Washington existed. I would still know about his existence and believe in it, but I would also believe an untruth about him.