I don’t think there is any value to the underground facility claim, as I said, the style the WTBTS likes to build in requires all facilities to be underground. Bigger buildings require larger setups. Commercial entities generally put things in the building or on the roof, but that requires a lot of highly skilled work and project planning, things commercial entities care about since they pay per hour (30-50% of a building construction cost is materials, the rest is labor). Building it underground gives you lots of time and space to make and fix poor decisions, especially if your workers cost nothing.
Hence why almost every Kingdom Hall has all their utilities under the ground without any access to actually replace them later on. The building is only designed to last 20-25 years and all the things that require a long time to setup are done first, that way they can claim a very “quick” building setup, rivaling that of commercial entities but done entirely by “volunteers”.
But all those people that are supposed to remain silent are skilled people that will work months before the event setting lots of things (water, electric, sewage, HVAC) up, and work months after dedication to fix issues after.