Either there were transport networks to bring in food from outlying areas, or the spaces between the structures, or terraces on them were utilized to grow food (or a bit of both).
Tens of thousands of buildings in a single swath - That alone is a hundred thousand people. Many children and multi-generational dwellings, as these things go... Assuming adjoining LIDAR sweeps would have the same results - however many sweeps beyond the adjoining...
That much population density definitely requires supply (it is too big for the land it is on to feed it)... Large acreages of farms must surround it... And those farms require two things that should be evident... Roads and water.
Roads are just as easily apparent as foundations are - Just the compression from use does not easily go away, not to mention gravel or corduroy leaving evidences. And while ditch irrigation to feed fields may not survive, canal-ways and feeder systems should be evident. Likewise sewage facilities. You just don't get that big without these things.