According to the dictionary "Illusion is something that deceives by producing a false or misleading impression of reality".

The question here is who produces the decision, for us the "reality" is that we (as persons) take a decision, according to the article the one producing the decision is the subconcious not us (of course we can argue if the subconcious are us or not, but I think that's more a metaphysical/psychological discussion).

So the brain deceives us to think that we are taking the decision which produce a false reality (or impression) and not the brain itself.

So for me the word is used properly. Altough I agree with you, illusion is often not properly used word.