It's congress as well, they can only call witnesses or documentation for purposes of reviewing/updating/creating legislation. So it is only even a valid summons if they are saying they want to create legislation that investigates the executive branch members and their connections to root out corruption. Thus it would make more sense to summon Trump and Bannon as well for their current ties to the executive branch, rather than Bill, who has not been a member of executive branch for 25 years.
Congress writes legislation, it doesn't judiciate anything.
It signals nothing