I'll take back the imagination part. Maybe I read more meaning into it, but I give the benefit of the doubt that you're not trying to imply incompetence without basis. But, respectfully, I feel it's still in the wild speculation camp.
To me a far simpler explanation is that, like any major project with a deadline, there are short term goals lining the process. One job needs more time here, it pushes back the start of another job, so maybe pull someone off this part to make up time, etc. Eventually, you see that you're missing more and more marks, and it comes time to recalculate your projections. I would agree that it's been in the backs of their minds for a while, but until you reach a point where you have to make the decision, you proceed as if you're going to make it.
I just find that far more plausible than them getting blindsided my any particular thing, least of all 300 people missing something a handful of testers catch. That's just from my experience managing large projects. I don't work in the industry, so maybe what you described is more common than I'd think, so I won't dismiss it. Just seems like it's reading too many leaves for my taste. Not everything has to have a single cause. A project this size with this many moving parts is bound to have to reevaluate the timeline that was created 3 years ago.
But, I do apologize for jumping to conclusions. The internet is full of some crazy during times like these, so I had an itchy trigger finger.