To give a little colour around the THC thing. I take a tincture solution in the morning for pain that would put me a little shy of 4 ng/ml, I'm totally safe to operate vehicles (although I don't). If I smoke a pretty big joint, I'd be at around 12 ng/ml - additionally, the decline of THC in the blood is pretty rapid.
It's pretty hard to know for sure, but if I had to guess I'd say the pilot smoked a fair sized joined either directly before flight or maybe even during given the closeness of the carboxylic and non-carboxylic.