There's no way to answer the question because you haven't defined the word "truly." Does it mean "perfectly"?
A person in the state of mortal sin cannot receive the Eucharist. He still has the obligation to attend Sunday Mass--i.e., to worship God. I conclude that he CAN and DOES worship God, imperfectly.
What I learned is that it is possible for men to know about God through the use of human reason unaided by supernatural grace. Men can come to know God through natural revelation, and, indeed, all men are obligated to know God in this way, in proportion to each one’s intellectual capacity. I think that when you talk about the moslems believing, it is the God they can know through reason.
But it isn't always black and white. The Latter Day Saints believe that they know God, and even baptize in the name of the Father, the Son, and the Holy Ghost. But the god[s] that they identify doesn't share all that many of the attributes of the true God who is knowable by human reason. The god they identify is not infinite, all-knowing, all-powerful, all-present, eternal (at least as we mean it), supreme, almighty, or even a trinity as we know it.
Yet, I've known LDS folks who seem to have drawn close to Jesus in their prayer life. Interestingly, some of these that I've met seem ultimately to de-emphasize the false ontology of the LDS god, almost as if the true God is drawing them close, and causing them to slowly shed their false beliefs about Him. I speculate that perhaps these become Christians through something akin to baptism of desire.
The question is, do the moslems believe in the one God? They seem, formally, to believe many things about God that identify the one in whom they believe as the one God - all-powerful, supreme, eternal, transcendent.
Many of the things they believe or don't believe are not so much errors as omissions. They don't believe in the Trinity. They don't believe in the divinity of Jesus.
In many ways, this puts them in a similar situation as Jews, and I wouldn't say that the Jews don't worship the one, true God.
But there are a few things that moslems believe that are not so much omissions of believe but outright misidentifications of God. They believe that God can and does lie. But that would mean that God is not Truth. It would mean that God is not perfect, and thus, is flawed, incomplete, and in need. They do not believe that we are made in the image and likeness of God. But that is to deny, then, that God is rational. It means that God is capricious, and thus, untrustworthy. They believe that God authorizes the moslems to rape, murder, pillage, plunder, and enslave generally. But that is to make God an accessory to murder, adultery, theft, covetousness, and every manner of evil. It means that God is immoral, that he is not holy, he is not morally perfect. Frankly, the god of the moslems seems more like, in some way, one of the gods of the Greeks or Romans, with many of the flaws of sinful humans.
Of course, the difficulty is that there is no moslem magisterium, and thus, it's difficult to nail down just what islam is - what is “true islam.” In that islam is an utterly false, satanic religion given to mo by Satan himself, I'm inclined not to even concede that there is a “true islam.” Just a pile of landfill waste masquerading as religius faith.
Nonetheless, I've known a fair share of moslems in my life, and not all want to behead us, rape our women, enslave our sons, or throw us in cages and set us alight. There are a few moslems who wish to live in peace, worship the God who created the splendor of the universe, who put more emphasis on the mercy of God, and reasonableness of God's actions. I've known a few like this.
It's difficult for me to look at these people and say that they worship a false god, as it appears to me that they seek Him in sincerity and good faith. To me, it appears that they seek the true Bod, but through a false religion. They worship the true God, but falsely.
All this suggests to me that moral atrocities like ISIS, and the hundreds of millions of moslems who desire strict sharia, who give their assent to murder, rape, pillage, plunder, rape of little boys, adultery with married women, are, indeed, pagans, worshiping a false god of their own creation, or even of demonic origin, while the small remnant of moslems who seek the God of mercy, the God who is just, holy, and liberating, who eschew that God commands all manner of evil, merely worship the true God falsely.
sitetest