It seems weird that they don't mention Nick Redfern's book on the topic where I believe he did a lot of the original research into the hallucinagen hypothesis. IIRC, it was moreso that it was an MKULTRA style experiment on British troops, not a naturally occurring, incidental EM plasma.
https://www.amazon.com/Rendlesham-Forest-UFO-Conspiracy-Government/dp/1945962283