Fox guarding the hen house?
The hack not only directly networked to servers over the Internet, the hack also networked - spawned - to other areas throughout an organization affected - thus creating multiples of compromised hosts that might have alternative / backdoor communications over the Internet.
Among the hack objectives, was to locate files and delete the files - by removing the paths to the files on Windows OS systems.
Yet, another hack objective, was to set up Wide Area Botnets.
A security feature of the (SolarWinds) Orion Platform, aboard a customer device, is claimed by SolarWinds (if I have that correct), to have indicated a problem that led to SolarWinds hitting the alarm.