The summary says the main question addressed is whether synesthesia is a brain disorder. But that's a more loaded question than what the paper actually looks at. The paper only asks whether it's a neurological condition. Without too much nitpicking I'll say if you ask whether it's a disorder, you're accidentally pulling in a question of whether synesthesia causes suffering, which is irrelevant. All the cited paper investigates is whether synesthesia is detectable at all by physically measuring the brain as opposed to relying on subjective experience.
Aside I have to admit I am confused by the word "neurological". If a condition is neurological does that mean it is rooted in the nervous system (which includes the brain)? Is a condition that is rooted in the nervous system, but we do not yet know how to observe or study it, still "neurological"? That seems like the case for synesthesia.