I want to ask the girl, will you be angry if your boyfriend doesn't reply to the text message in time?

Yes, it's normal to be angry. It's a feeling of being ignored.

But it is also possible, as he explained, that it was very late when he saw the text message.

For such people, it is best to call directly. My friends and I don't like texting very much. Text messages are silent, so sending text messages is generally not urgent and irrelevant.