Waltke didn't state it. Dr. Poythress stated that he did, here ---> GENESIS 1:1 IS THE FIRST EVENT, NOT A SUMMARY.
Here is the conclusion:
In conclusion, all three of the main arguments for the summary view have superficial plausibility, but none has weight. In addition, as of 2001, Waltke himself no longer holds to the second argument contained in his earlier work (in 1974 and 1975). The summary view is much weaker than many have taken it to be. By contrast, the initiation view makes good sense of the phrase meanings, theology, and syntax of Gen 1:1–2 in relation to Gen 1:1–2:3 as a whole, and beyond (the rest of Genesis and the rest of the Bible). It is the correct view. "
As you can see, Dr. Poythress has reason to believe that Genesis 1:1 interpreted as an event is the correct view. Waltke's second argument is a premise about an implied organization within Genesis 1:2 that doesn't hold.