It’s not silly. Look at the evidence of FBI entrapment over the decades. The Gov. Whitmer case the most recent example of a dozen fed agents entrapping a handful of morons. It’s their most successful MO; have agents recruit and activate sub 85 IQ morons into some sinister plot and then claim they busted the world’s most sophisticated and nefarious criminals.
Agents are FBI employees who are placed into groups such as ANTIFA or militias. These are the true "undercovers."
Sources are private citizens who are members of these groups or have affiliations within these groups who provide intel to agencies like the FBI. This usually happens when the source is busted for something and provides intel to the FBI in order to get out of trouble. Or because they genuinely believe the group is dangerous and want it stopped.