I think before you make this conclusion, you have to say something about how one is supposed to pick the weights.

I agree with this concern. The theorem is basically saying that, given any sensible aggregation rule, there is a linear aggregation rule that produces the same decisions. However, it assumes that we already have a prior; the linear coefficients are allowed to depend on what we think the world actually looks like, rather than being a pure representation of values. I think people, especially those who don't understand the proof of this theorem, are likely to misinterpret it.