The argument is indeed so bad, all it proves is that the church was so powerfull at the time that they could spout any ludicrous nonsense and still have people bow in obedience.
The argument was created by a churchman for churchmen, it wasn't intended or used as a popular argument, but it's worth understanding that c. 1000 CE this argument seemed compelling to literate folks who were introduced to it. If people had thought it was ludicrous nonsense they actually were free to say this, this is a philosophical argument, not doctrine, and the Church didn't have too much to do with supporting/opposing such things other than opposing philosophical arguments seen as heretical.
And even Aquinas, who did not think Anselm's argument worked, thought it worthy enough to engage.
At the time, in Europe, atheism was simply not a permitted intellectual path to take. It was basically inconceivable. So nobody truly needed to "prove" God, and the argument "works" for some thinkers because its presupposition and ends confirm what they already "know" to be true.