Maybe I got lucky in thinking about/writing the solution, but it took literally 1 minute to finish this in Python. I think it would be better classified as a 7 kyu or 6 kyu problem

Two minor suggestions since this is probably targeted to people new to coding/codewars:

1- adding a breakdown of one of the sums, like:
sum(5) = 1 + 2 + 3 + 4 + 5 = 15

2- adding to the end of the description the largest number tested, and a disclaimer that if the solution is timing out, the code needs to be reviewed or a non-brute-force solution has to be thought out

Huge coincidence: I've just been through a few katas from that series and your comment helped me figure out this one (not a big math guy)
Thanks!

Empty lists are considered as False by Python. Using a not reverses that logic and enters the if statement when the list is empty

https://docs.python.org/3/library/stdtypes.html#truth-value-testing

YEP, WOW