"Take 2 strings s1 and s2 including only letters from a to z."

"but still this solution seems to be correct, because task guarantees that only letters will be passed in. solution does not have to check for non-letters."

That is fair I suppose.

• solution has to be correct to be posted here.
• any solution is just as correct as far the tests go. If tests are poor, invalid solutions can slip through.
• but still this solution seems to be correct, because task guarantees that only letters will be passed in. solution does not have to check for non-letters.

I thought a solution had to be correct to be posted here?

This is incorrect. It does not test for non-leters.