Not exactly truth - in fact only half true. 
Saunders said something like (the deep state theory is) "the imaginary product of wild eyed conspiracy theorists" - your words.
He most certainly did NOT state or even imply that it actually existed.
He said, "If you see the public sector as a partisan political entity, then the inevitable result is that YOU (the conspiracy theorist - my capitals) turn it into one".
Presumably by attacking people as an identifiable target group.
I wonder what Trump means when he refers to The Swamp?