> Back in the 1990s, it was exceptionally difficult to get TS clearance. SCI on top of that must have been even harder.

I think you are exaggerating this as the other poster points out. Every US military cryptolinguist gets a TS-SCI clearance. So, while every student at Defense Language Institute was doing his/her language course, a background check was done, and the rejection rate in the 1990s was presumably tiny. And ditto for other military intelligence roles. A certain amount of military personnel getting the clearance were semi-native speakers of a language in demand who still had foreign ties (family or land owned in the old country) but they passed nonetheless, which must speak to a certain laxness.