Deists believe that god is not necessarily knowable and that all we can really know is how the natural universe operates.
Based on what evidence do they conclude an unknowable God?
That is not a god that can be proven: it is not a scientific hypothesis, it is not intended to be one. It is based on faith not logic.
That the claim has to be taken on faith is sufficient to refute it for reasonable people. It's always reasonable to reject conclusions that are developed without any recourse to evidence. That which is
made up is almost never true.
Just like a book made up out of random words is not likely to describe the accurate anatomy of the human body, conclusions based on making things up are so astronomically unlikely to be right that we can reasonably reject such endeavors wholesale.
Surely no reasonable person can dispute that making things up is not a path to truths.