Commentary on Tafsir al-Jalalayn: ((Inkum)) contains a shift in address ((for they shall taste the painful punishment)).

Then He, the Exalted, said: There is a shift in address here. Yes, he says there is a shift in address. The shift in address means that the context requires saying "Inhum" (they) shall taste the punishment, because the entire narrative was in the third person. So the context required saying "Inhum" (they) shall taste the painful punishment. But there was a shift in the context from the third person to the second person. What is the benefit of this shift? We mentioned previously that every shift in address has a common benefit, which is to alert the one being addressed, because if the speech remains in one mode, the listener or reader may become inattentive. But if the style changes, he becomes alert to why it changed and what the point of the change is. All shifts in address share the purpose of alerting, and then each instance has its own specific aspect. Here, the shift is from the third person to the second person because the second person is more effective in warning. Therefore, if we consider the story of al-Khidr and Musa, when he first reproached him, he said to him: And the second time, he said: So the second person address undoubtedly strikes the mind directly, making it more impactful than the third person pronoun. The rest will come, if Allah wills.