To make the point more clear.
Just because Allah allowed humans to change the message which was taken to be as His word on Earth, doesn't mean that Allah was powerless to stop that.
It is the same as like when God allows all the suffering in the world, it doesn't mean that He is powerless to end it. It is according to His plan for humans.
The truthfulness of whether a particular alleged God is the actual God, depends on testing the promises that He makes. As far as I know, Allah never promised that all the divinely inspired books, in the hands of humans, will remain corruption-free till the end of time. Therefore, if such happens, it cannot be used as an argument for His refutation. This is according to, what is apparently, His plan for the humans.