The reason why iphone6plus can accept SMS in airplane mode.

IPhone6Plus can't receive the signal in flight mode. If it receives a signal, it is not in flight mode.

So that message you receive must not be in airplane mode. Because there is no signal at all when the flight mode is started, how can I receive a text message? You can check your mobile phone settings. Maybe it's WeChat instead of SMS. The reason why you set the flight mode from the beginning is because the plane can't be disturbed by radio waves when it takes off, so you can't receive radio waves when you set the flight mode. Which is the signal.