There is an established test for midichlorians, that doesn't mean that their theory that Mid-count is the cause of force sensitivity is some kind of fact. It's just a likely theory.

The idea that Midichlorians are sentient at all is a belief that Qui-Gon has.

The coutnerpoint is that Sidious claims that Plaguis or even he knows how to manipulate the midichlorians to create life.

So, the idea that the Midichlorians have a will to create Anakin or even that the Will of the Force created Anakin is now in question.

I think Lucas is just putting the Midichlorians, and the theory they are the cause of Force Sensitivity into the film simply to present that scientifically they have been able to draw a co-relation between midi counts and force sensitivity. I don't think he means to say that any of the theories they have come up with based on that is somehow "the way it really is". It's just a theory.

It's up to you to wether or not you believe that it's some kind of conclusive fact.