The theory that the old underground trolley/streetcar system tunnels run underneath Comet Ping Pong is incorrect.

I found a map of the old underground streetcar network and the Connecticut Ave branch ended just south of the Smithsonian Zoo. Comparing the steetcar map to Google Maps, and using the measurement tool makes the old tunnel end 2.5 miles too short of Comet Ping-Pong.

1948 Steetcar Map

Also, I used to watch a lot of DIY and construction shows. IMHO, the work digging up the concrete floor is to repair/replace sewer lines. You can see the original concrete, the soil, the fresh gray aggregate to level/bed the pipes and the PVC pipes themselves. The hole dug against the wall is either a) a water seepage issue from outside where they intend to repair the moisture barrier. b) the foundation under the brick wall has cracked and needs underpinning to stop it getting any worse.