Land and "improvements" are assessed separately, and I believe this is plotting just the assessed land values. In the small text about each map, it says to use the settings to switch to full assessed value or improvements. But still, it's very hard to actually assess land value in an area like Manhattan where there are basically no land-only transactions

It is a good question. The author seems to use the field "assr_land_value", and there is also an "assr_impr_value". So it very well may be correct.