Those with deep fear (khashi’uun) are those who think that they will meet their Lord and that they will return to Him.
Allah U says ﴿يَظُنُّونَ﴾ “think” and He did not say (متيقّنون)“are certain” while the word “think” in this case actually means “are certain” so why did Allah U use the word “think” while these people have certainty?
This is a great sign in the Holy Qur’an. If it was a man who wrote this Qur’an, it would never occur to him to write “think” for as long as the intended meaning is “are certain”.
The meaning is that they are certain. However, for them, just to think about it causes them to rush to obedience. An example for this would be if you were on a path and you came to a fork in the road. And you can choose which way you want to follow. If someone says to you, while pointing to one of the paths, “I think there might be highway robbers on this path” you would immediately take the other path simply by assumption, that is, you think it is safer.