The biggest let down with Endgame was not Fat Thor. (Spoilers)

[80sbaby]

Were people not paying attention or something?

I don't think it's so much they weren't paying attention as, assuming they already understand time-travel due to other films. Some audience members likely made the same mistake as Rhodey and Scott. So when Banner was talking to the AO, they misheard a few things and went with their own understanding of time travel subconsciously.

But you are 100% correct in your explanation of how it works in the MCU. Nebula proved that.