The article specified a list of positive integers, and you can’t go smaller than the smallest initial value.
It also clearly shows right in the first example of the first rule that you can go smaller than the smallest initial value: [7, 5, 3] -> [6, 1, 5, 3] is the first legal move it shows, and you'll notice that 1 is smaller than 7, 5, or 3.
If you split 5 into 6 and -1... 6 ain't smaller than 5.
* You can never make an integer greater than the largest integer in the original list.
* You can never make a move that results in the same integer appearing in the list more than once.
So if the original list was [10, 1] and negative numbers were allowed, then [10, 2, -1] would be allowed.
1. Split an integer into two smaller integers.
2. Combine (add) two integers into a larger one.
Both suggest that negative numbers are not possible.