One organizes many contributions using any number of industry-standard design methodologies. Designing airplanes and cars uses even more engineers.
I suspect NVIDIA is slightly exaggerating and are counting the contribution of many "overhead" engineers that provide value for the whole engineering organization, such as people who work on design tools, design kits, methodology and the like.
You're right, there are many repeated subunit but each unit needs a team to be optimized.
For a chip this complex you need:
Logic Designers (who come up with high-level models for the chip and define the instruction set / hardware interface)
Front-end engineers that write Verilog and/or VHDL (I have no idea what NVIDIA uses)
Implementation engineers (who do place and route and parasitic extraction)
Verification engineers (who use various tools to see if everything is as it should be)
Packaging engineers (who work closely with vendors to develop a custom package for the chip/module)
Module engineers (since we have 3D stacked memories on this device the module engineering is far from trivial)
Thermal Engineers (3D modules typically have very complex thermal requirements)
Signal Integrity engineers (since we're going so fast just getting a signal from point A to point B is hard)
Analog/Mixed Signal engineers (for clocking, serial I/O development)
Integration Engineers (for modeling how to put all this together)
System Engineers (for figuring out if this is all going to work)
Software Engineers (for low-level software dev)
CAD Engineers (for developing and maintaining an appropriate computer-aided design flow)
Foundry Engineers (for working with the foundry on the physical production of the wafers... anything this big and complex will need process customization)
ESD engineers (for figuring out and implementing an ESD strategy)
Library Engineers (for customizing and optimizing the standard cell library used in the chip)
Product Engineers (for solving production problems as they arise)
Test Engineers (for developing and implementing tests to show the chip is working as expecting)
Application Engineers (who work with early adopters to integrate this chip into their systems)
and on and on and on...
As you can see, an army of engineers is required for a chip this complex to see the light of day. On simpler chips, many of these roles can be played by the same people, but in a chip this big, they need to divide the work or it would never get done.