Your point is right. The correct way is > The most amazing thing **was** the people who... I tried to think about the other specific possibilities. Sometimes we use "were" in the case of the subjunctive mood, however, it is used after "if, as if, wish, suppose". Here is not that case except if it is like > (I wish) the most amazing thing **were** the people who... "I wish" was not said, but meant.