That tool seems interesting, but it doesn't match the seat map presented by KLM.com at all for this journey in terms of +/- or X/!X. Lots of discrepancies. It does show seats that have already been assigned, but doesn't differentiate between seats that show up as selectable or unavailable (and unoccupied) on klm.com.

I guess I'll have to do some field research to confirm my theory that KLM is trying to spread people out while also leaving bigger spaces for groups.