It's the other way around: the Zeeuwse Legion was composed of the 2nd and 3rd bataillon of the 5th Infantry. In 1808 the 5th regiment was reduced to a single bataillon. I don't know whether this meant that the Zeeuwse Legion was separated from the 5th regiment, or reduced.

The Zeeuwse Legion was to be completed from native men. The reason for this was that Zeeland (the Isles) had a very unhealthy climate. It was riddled with malaria. Native men were felt to be more resistant to the disease.