I wonder what they did to ignite all of them at the same time? The normal Estes rocket electric matches have pretty poor timing in my experience.
The power delivery setup for this launch was probably the most interesting bit. Estes igniters are typically rated to require 2A. Supplying that times 5000 is no simple task. The video seems to show five large boxes (~1m on a side), each surrounded by a cluster of rockets. These boxes probably each concealed a pile of batteries and relays.