Former individual contributor, former middle-manager and former "VP-level boss" here. I've seen all this before, several times over.

Know that this will recur throughout your career, most likely. Your boss is disconnected from the project and the manager, and the boss has no idea what's happening at the individual contributor level. It's always "you shoulda done this, you shoulda done that" It's always in retrospect from a boss like this, never, ever in anticipation. Did the manager or boss say: "Hey dazeandconfuse, we've seen this before, you can look at how the Widgetspinner project solved that issue for guidance." No. They're not there for you. They're there to blame you when their failure to support you manifests itself.

I'd bet that the manager and the boss don't have productive 1-1 sessions on any regular basis (or even very good communication) and thus the boss doesn't know the status of things. The boss ultimately is too lazy to care or really be involved, doesn't listen, or the manager sugarcoats things, or lies outright, or a combination of all these things. No one is accountable on a day-to-day basis. The boss never, ever looks at the project plan with any comprehension - even though they're constantly insisting you document stuff that they'll never read. Especially action items from meetings.

In cases like this, the boss will pretty much always over estimate how quickly things can get done, and under estimate how much work and complexity is involved, or what the real bottlenecks are (and in all likelihood that the boss is a big bottleneck themselves. They are overconfident in the resources they have, and probably don't institute positive processes for a happy, productive workplace. The boss will ignorantly sabotage things by withholding information, failure to be involved, fail to communicate, fail to have clear goals, failure to identify outcomes, failure to see the big picture.

We used to have a phrase about changing requirements, the case of "The Executive Drive By". The boss will make a suggestion for an enhancement: When that enhancement changes the schedule and delays a project then "It was only a suggestion, I didn't set a new requirement", or if the project stays on time without the suggestion then it's "Hey you guys screwed up, where are the blinkenlights I asked for, can't you guys get this right? Who is running this project?!?" Win-win for them, lose-lose for you. Just like they want it to be.

I'd take it as a signal of poor communication up-to and down-from the boss, and in all likelihood the boss isn't really interested in ensuring the manager's success. If the boss was unhappy with progress, the manager should've known long ago, before the deadline slipped, and it should've been communicated by the manager down to everyone with new expectations and goals and a plan that eveyone was on board with -- including the boss. That that didn't happen tells you everything you need to know. There are problems. Probably systemic, and unlikely to change.

It's a sign of typical dysfunction and the natural adversarial relationship between manager and boss. Eventually the boss will seek scapegoats down the line when things don't go well, never accepting any blame themselves. Never.

Eventually won't go well enough that a "reorg" will happen and people will be shuffled around, someone will get fired, and ultimately nothing will change. I've seen it happen over and over again.

Also, know that you don't owe your company jack shit. They are not, and will not be loyal to you, ever. Ever. They will cut you at first necessity. Be prepared to do the same at any time. Boss will blame manager, manager will blame you, people will get fired, unjustly or not. Probably unjustly. Bosses never fire themselves, ever. They will always place blame down the line. Always. Bank on it.

PROTIP You're new in your career: As long as you are employed, powersave a portion of your income into a separate savings area as F-U money -- so you can go 6 months or more that you can live on in between gigs. This gives you ultimate power over the boss: "If you don't like my work or my performance, F-U, I'm out."