This study doesn't establish that an odor precedes a seizure. They took the samples from someone who was in the middle of a seizure or shortly afterwards.

This study doesn't establish that, no. But there is considerable anecdotal evidence of service animals that can alert well in advance of a seizure, before the patient is aware of any prodromal symptoms.

(For example: https://news.ycombinator.com/item?id=19522755)

That's interesting but how do we know it due to smell rather than sound or other senses? I've read that dogs and other animals can also anticipate earthquakes because they can hear or feel the earthquake earlier than humans.

It is a strange coincidence however that the seizure chemical is the same chemical that causes a dog to bark.

It’s probably just that the dogs can detect a problem and are barking out of empathetic concern or fear (if not trained)

Well it's odor they can detect and distinguish, and then can be trained to either bark and or take other actions.

Dogs don't need a chemical to make them bark.