You did that problem because your reagent is sodium hydroxide. In order to prove that the neutralization reaction really happened, you need excessive hydrochloric acid, which can be shown by pH value. Therefore, when the pH value is less than 7, it can be said for sure that sodium hydroxide and hydrochloric acid have reacted, which is a neutralization reaction.

As for the neutralization reaction, it doesn't have to reach a certain proportion to happen. As long as there are hydrogen ions and hydroxyl ions, neutralization reaction will occur, but sometimes the reaction is too weak to be ignored.

There is no saying that "the two solutions are diluted if they don't reach a certain proportion together". This statement is wrong.