Little squares' (3x3 in example above) may also only contain integers: 1..N (N included)

plz, where is this rule in the description?

Because the 3x3 little squares have repeated numbers.

[1, 2, 3, ...
[2, 3, 1, ...
[3, 1, 2, ...

Why this test: 'Sudoku with invalid boxes (little squares), but valid rows and columns' fails on these data?
[[1, 2, 3, 4, 5, 6, 7, 8, 9],
[2, 3, 1, 5, 6, 4, 8, 9, 7],
[3, 1, 2, 6, 4, 5, 9, 7, 8],
[4, 5, 6, 7, 8, 9, 1, 2, 3],
[5, 6, 4, 8, 9, 7, 2, 3, 1],
[6, 4, 5, 9, 7, 8, 3, 1, 2],
[7, 8, 9, 1, 2, 3, 4, 5, 6],
[8, 9, 7, 2, 3, 1, 5, 6, 4],
[9, 7, 8, 3, 1, 2, 6, 4, 5]]

got it

Read the instructions again, it's explained there in detail, 11211 is an interesting number.

// progress as we near an "interesting" number
isInteresting(11207, []); // 0
isInteresting(11208, []); // 0
isInteresting(11209, []); // 1
isInteresting(11210, []); // 1
isInteresting(11211, []); // 2

The values inside the array could be ignored here (about the test you're not getting), because you're far away from them.

what is 11211 value?

Because 11211 is 2 steps away from 11209.

Test.assert_equals(is_interesting(11209, [1337, 256]), 1);
why 1 is a correct result?