They might actually know for much longer: if your spying devices suddenly stop communicating to you, that's likely you've been discovered.
If that story is true (and I personnaly think it has a high probability to be), what would a gov or a large org do? Investigate, confirm they have been compromised but then.... leave the hw in place and data flowing back to the alien mothership? Unlikely.
Now you can observe them and only intervene when absolutely necessary, thus giving you time to learn more about the attackers and their methods.