From the Wiki:

If the attacker has the majority of computer power, "the attacker can't reverse other people's transactions without their cooperation."

I don't quite understand why this must be the case. Let's say attacker wants to undo a transaction k that happens at block i. Why can't an attacker with a majority of hashing power start mining a block j originating with block i-1 that includes everything but k? Then, the attacker will succeed in the long run and the transaction k no longer appears in the longest chain, right? So I'm not quite sure why you cannot reverse other people's transactions in a majority attack.

Diagram for clarity:

     -i(includes k)
-i-1
     -j(does not include k)...(this chain should be longer w/ majority computing power)