HOW?!

Thanks ;-)

I have posted a better version with explanation here [https://www.codewars.com/kata/reviews/6001e813dd245e0001e32303/groups/601f0be36aef7e00019452c0]

Could you explain a bit about this compression method?

None means that your function did not return anything. If your function would have, for example, returned the number 3 the output would be: "3 should equal [2]"
You can return an empty list with:

list = []
return list

The strings that tests expect are all simple and without caveats.

The strings that your functions return you may print yourself in whatever way that looks appropriate to you. You may even print them as hexlified bytes. I would consider it a part of the kata. :)