Doesn't an exercise like that come with a high risk of getting shot?
I am guessing he intended to surrender immediately when challenged, he just never was. It's a civilised country, so we tend to not have have shoot-first-ask-questions-later policies, at least not while I was pulling guard duty.
This is entirely reasonable, as there's not that much damage you can do between being challenged and surrendering (or not as it may be), at least at the fairly mundane mechanised infantry battalion I was stationed at. What are you going to do, steal a tank? Good luck with that! (Go for missiles instead!) I guess things might be a bit tougher at the base where they house the American nukes. If it is, I would presume there to be very obvious signage.
I do remember the story of a guard officer trying a bit too hard to test his troops and not surrendering when challenged repeatedly (different base). He was shot in accordance with the rules of engagement. ¯\_(ツ)_/¯
Regarding nuclear facilities, that reminds me of story on juridiction between an US Marshal and the security personnel of nuclear facility.
http://www.funraniumlabs.com/2018/09/the-fringes-of-regulati...
Nuclear bases in Eastern Russia are all built with though of stopping, and stalling a substantial military assault from paratroopers, sabotage specialists battalions, or forward armed recon that might preclude a full scale conflict, but security was known to be lax (thought still way above any normal base)
And in US it seems, they are more wary of infiltration by small force, or "trickery," or, I'll give myself liberty to imagine, enemies from within.
You’d think, but if an 80 old nun can get in and et a selfie with a nuke, it can’t be too hard ;)