Well, you've got two possibilities for what happened. One, the base commander (or whoever gave the presentation) had a sense of humor and the joke
got past the reviewers (who may well have been the base commander).
Second, the guy in charge of the powerpoint presentation actually ahs advanced knowledge of Reticulan-Navy relations, and the censors let him tacitly
admit to that with a poorly compressed MS Paint UFO.
If you're thinking that they
might censor a joke about aliens, they should definitely be censoring the truth about them