https://www.nytimes.com/2019/10/02/us/politics/adam-schiff-whistleblower.html
The WB was not satisfied with the way the CIA was handling the complaint, so he ran off to congress.
"The C.I.A. officer approached a House Intelligence Committee aide with his concerns about Mr. Trump only after he had had a colleague first convey them to the C.I.A.’s top lawyer. Concerned about how that initial avenue for airing his allegations through the C.I.A. was unfolding, the officer then approached the House aide."
Your narrative about the CIA only learning about it after the ICIG seems to be incorrect.