CONUS used to mean CONtinental US, back in the day, from a US Army / military perspective.
I found several references to that 'continental' meaning but that excludes the state of Hawaii. There are other references to 'contiguous' so the usage would also include Hawaii since that state is not a member of the set of states that are present on the North American Continent but is conceptually sharing a border with the rest, so is in concept contiguously sharing borders and therefore part of and inside the borders of the US.