
Jake Z. answered 05/06/19
Experienced TA & Tutor for Computer Science (Java/C/Python)
Technically, you are correct, as when 1 goes to infinity it will dominate. However on a smaller scale, the difference would be noticeable. O(m+n) makes it clear that the function scales linearly with m as well as linearly with n, where n & m are 2 distinct datasets/parameters/etc.
Hope this helped!