They were planted there as a ruse to get a FISA warrant which would allow surveillance of every person that the dupe communicated with. That would involve many, many people in the Trump campaign or White House.
I have read that a level 2 type FISA warrant would allow surveillance of every person that the communicated with person had communication with but I cannot find verification of this. I think that it is even retroactive. It is an outrage.
I believe you supposition is right on the money. IMO there is zero chance the enigma did not know about this.