Is saying, "By Allah, you will be held accountable for that," to someone who interprets the Quran without knowledge considered speaking about Allah without knowledge, even if the intention is that Allah will hold him accountable for his actions in general, and what is the repentance in this case?

Swearing that a specific person will be held accountable for a sin is speaking about Allah without knowledge and presuming upon Him, because the fate of a servant is subject to Allah's will. He may repent, or Allah may forgive him. Some people may even enter Paradise without reckoning. One must repent from such an oath by seeking forgiveness and performing righteous deeds, for the door of repentance is always open.