Did you play ac4? Because while 3 was the low point of the series 4 was probably the best, or at least the best beside ac2. And that's universal opinion. Nothing to do with people "learning their lesson"
To be fair, he said "in my humble opinion". AC3 was a letdown, yes, but not the series lowpoint - "universal opinion" would be that it was Revelations - many consider it a filler game and I didn't see any positive feedback for den defense. While I agree that AC4 is one of the best overall, it did fall short in some areas particularly the main story, main missions and Assassin combat. I'd say it was best as a pirate game with some great side missions like the Templar Hunts and Assassination Contracts.