The article could be located here.
Jochen presents three different arguments. One from Surah 21:4, the second from Surah 23:112 and the third from Surah 20:121-128.
In Surah 21:4, I really don't see the argument. The verse is clearly and most likely in the past tense.
In Surah 23:112, the verse is in the past tense because Allah is not bound by time as we are and He is showing us that He knows what is going to happen. Also, the context is clearly speaking about the Day of Judgment. So Allah does not have to repeatedly say "then this will happen and then that will happen"
Similarly, Allah uses the past tense for His Coming on the Day of Judgment in Surah 89:22 and in 19:30 indicates that He has given Jesus revelation while he was a baby.
Why is that so?
Imam Qurtubi cites several opinions of the meaning of Jesus' statement and the strongest one is that Jesus was intending to say that God has decreed that the Book (Gospel) will be revealed to him. So it is in the past tense in the verse since it is referring to the act of decreeing.
Similar figure of speech is found in Surah 108:1, where God says that He has given (in the past tense) the Prophet (peace be upon him) a river in heaven. Obviously, once the verse was revealed to the Prophet (peace be upon him) he was still on planet earth and didn't have the river in his possession. However, the verse is trying to signify the reality of Allah's promise and decree.
So we can apply the same logic to any verse that uses the verb in the past tense when speaking about a future event. It is emphasis on Allah's decree of what will happen and that He is speaking from His perspective.
It is a surprise why Christians would have a difficulty with this when they themselves believe that certain Old Testament passages such as Hosea 11:1, which is in the past tense is also referring to a future event (the coming of Jesus).
Thus, when Jochen Katz says:
Why didn't the author of the Qur'an use the future tense in these last two verses? Is that the clarity and eloquence of the Qur'an? I find it difficult to believe that using the wrong tense is more eloquent than using the correct one.
And he wants to ensure consistency then the Muslim personification of Jochen Katz could rightly argue back:
Why didn't the author of the Bible use the future tense when wanting to prophesize about the future Jesus? Is that the clarity and eloquence of the Bible? I find it difficult to believe that using the wrong tense is more eloquent than using the correct one.
Now Christians would argue back regarding the Hebrew language being different than modern languages regarding "tenses" (see here) just as we would argue back regarding the Arabic language. But as we all know, Christians like to employ double standards.