King County taxes land value as part of our property taxes. If Seattle up-zoned this property, that component of the owner's taxes would increase to reflect the value of its new highest and best use.
The point of a Land Value Tax is that everyone gets taxed purely on the value of the land and none of the improvements. So adding more units would only decrease the per-unit tax you pay.
Seattle here is attempting the same thing in broad strokes by literally taxing underdevelopment - but it's clearly not as elegant of a solution