It is true that the length of the new list is n-1, and therefore the slicing involves writing to memory only n-1 numbers.
In fact, if you wanted to be really accurate, you'd have to count also the max() operation, and the indexing into l[left], and perhaps other operations were are not even aware of.
The point is that all these operations take up a constant amount of work. Adding/subtracting a constant does not affect the node's complexity. Furthermore, adding/subtracting a constant from each node in the tree does not affect the total complexity either. So we shouldn't bother ourselves with these issues; accordingly, you are asked to specify only a tight O() bound on the amount of work performed in each node.