That test is like this:

test.assert_equals(sum_of_intervals([(0, 20), (-100_000_000, 10), (30, 40)]), 100_000_030)

The overlap is indeed between 0 and 10:
[-100_000_000, (0, 10], 20), {30, 40} so it's 100000020 + 10 = 100000030. Not a kata issue.