I have to concure with j.pihlgren, it is not clear what you are saying at all. If you are going to critique the solution could you please provide a better explanation as to what is expected as what is presented is not clear at all.

Moving on, failing any solution just because you don't like it limits the creativity of everyone involved including yourself.

If something solves a problem it is valid, period.

I found that out as well, just changed the test code to what it should be, but the actual test are broken as well for the last test I get

returns 'nopqrstuvwxy`abcdefghijklm' should equal 'nopqrstuvwxyzabcdefghijklm'

and the rules are that any special character should be ignored, but here it is not.

This kata instructions are flawed.

A friend of mine takes a sequence of numbers from 1 to n (where n > 0).
Within that sequence, he chooses two numbers, a and b.
He says that the product of a and b should be equal to the sum of all numbers in the sequence, excluding a and b.
Given a number n, could you tell me the numbers he excluded from the sequence?

So it gives us 26 as a test case, the sum of all numbers 1-26 is 351.

[[1, 175], [3, 87], [7, 43], [10, 31], [15, 21], [21, 15], [31, 10], [43, 7], [87, 3], [175, 1]]

Each number in this sequence can resolve the rules,

351 - 175 - 1 == 175

175 * 1 == 175

351 - 87 - 3 == 261

87 * 3 == 261

351 - 43 - 7 == 301

43 * 7 == 301

351 - 31 - 10 == 310

31 * 10 == 310

351 - 21 - 15 == 315

21 * 15 == 315

All the above are solutions to the problem but his only solutions are 15, 21 which is incorrect. Just doing the math on paper proves that. So the rules need to be revised because all the needed information is not provided.