Maintaining his level of clearance requires active monitoring, which means regular polygraph and investigations.
There are examples of traitors/ covert agents operating in nations on both sides of the Cold War despite their being "positively vetted" and psychologically profiled by the organisations that they infiltrated/ betrayed.
There are numerous examples of people knowingly defeating vetting measures. Polygraphs give false positives and false negatives, and can be deliberately subverted. At best they give an indication of physiological stress responses, which
can occur in many people who are knowingly repeating a falsehood, but not in all. Some individuals show indications of physiological stress when asked questions even though their responses are truthful and accurate.
(I don't think any of this applies to David Grusch- I have no reason to believe that he did other than provide loyal service to the best of his abilities).
I don't know what sort of questions are asked during positive vetting for the US defence establishment, but I would be surprised if they asked about people's views on UFOs. Vetting tends to be more an assessment of general background and character and if the subject, or significant others in their life, has or had any connection with "problematic" groups- extremist political or religious factions; nationals or agencies of certain countries; certain types of criminals etc. etc.
The belief that some UFOs are craft piloted by aliens seems to be held by many in the USA, but (AFAIK) it doesn't strongly correlate with political or religious extremism, work reliability or personal loyalty.
I don't see why people dismiss such evidence. Several of my friends have seen UFOs, and some of them are in the unmistakable category. I believe them of course.
People's perceptions can be fallible. That is why sceptics are interested in having evidence in addition to testimony.
I have absolutely no doubt that some decent, broadly healthy and generally reliable people believe that they've see alien spacecraft at close quarters. I always found the account given by the late forestry worker Robert Taylor convincing: while at work alone, near Livingston, Scotland, he saw a UFO about 480 metres (530 yards) away, two smaller spheres somehow seized him and were dragging him towards the UFO when he blacked out.
https://en.wikipedia.org/wiki/Robert_Taylor_incident
Taylor was generally accepted to be a reliable, no-nonsense sort of guy, and (I think) always came across as giving a detailed and consistent account of what he believed happened. Some of his account- and an earlier episode of acute illness- suggest that he
might have had an isolated epileptic-type event.
This might seem a rather dramatic explanation for his experience, which would rarely apply to other witnesses, but we now know that some people relate experiencing "classic" OOBEs (out-of-body experiences) and NDEs (near-death experiences) when they are not acutely unwell. (The parapsychologist Susan Blackmore had an OOBE, but has gone from belief in psychic phenomena to scepticism),
https://en.wikipedia.org/wiki/Susan_Blackmore
I suspect similar transient anomalous cognition may, very rarely, account for some of the more detailed and close-up accounts of UFOs (which are often "high strangeness" events).
But while the claimant may be giving a totally honest report of what they witnessed, without additional evidence it is interesting but not proof. As evidence, it is in the same category as appearances of the Blessed Virgin Mary (and visitations of religious figures to the devout of other faiths)- the account may be detailed and the witness a reliable person of great honesty, but generally I doubt that what they saw was objectively "there".