Counterpoint: even without lock tight secrecy, leaks can be ignored and covered over. Also there are certain secrets governments really have been good at keeping for a long time.
In and of itself, I don’t think this is a good argument against any particular conspiracy theory.
Make your MIT-licensed library big enough that the corpos use it, then switch it to AGPL just before you add a really important and tricky feature they’ve been waiting for.