Question about spy defense against revolt

[Taurean]

Playing on hardest, running communism.

I had a city with about 7 spies in it, most of them veterans. It was just taken over by an enemy diplomat by inciting revolt, resulting in me losing my entire attack force and about 1000 shields worth of production. Including all my spies.

I thought that having spies in a city defended against enemy spy and diplomat influence?

 

[Prof. Garfield]

I think diplomats and spies only defend against technology theft. Democracy is the only sure defence against bribery. I don't know how the AI responds to a large treasury or the presence of a courthouse.