This is true. When I said that both (A) and (B) were acceptable, I didn't mean to imply that they meant the exact same thing.
Welcome to the Grammar Exchange, by the way.
The exclamation point to which you draw our attention does not necessarily change the meaning of the sentence in any way, but I now see that it was probably meant to goad us toward (B) rather than (A).
Incidentally, I find the phrase "needn't have" less common in the US than in the UK, but by no means unacceptable. (In fact, I use it myself.) In the States, I think we would be more inclined to say " ... so I didn't have to (study/prepare) for it last night after all!".
Finally, I would like to add that I recently received correspondence from a very dear friend and colleague who is a native Englishman (a Cockney, no less), although he has spent most of his adult life in the United States. He wrote:
Looking in my old Concise Oxford Dictionary (COD) circa 1957, review and revise have somewhat similar meanings. We used in UK in my time “revise for an exam” for example, I don’t recall anybody using “review “.
For some reason, to my ear, revise sounds like a more thorough study of material for test, review suggests a less rigorous (UK: rigourous) look over the material. Probably just because of the usage from my past.
Thank you, Amalate, for your contribution.
PS: Extra special thanks to Mr Peter Campbell. You've helped me before, and I look forward to the next time.