This solution was written in Python 2. It does not work because Py2 is deprecated. It needs certain adaptation to Py3 to work with the test cases again.

Cool, would be even better if that would work for every case, it seems like it won't work anymore against the tests, now tests covers this case, I've just checked because I was curious ^^

solve in 3 lines what i spent 3 days to solve. respect!

3 lines. Respect

respect!

Yes, I think you guessed right. If f is an empty filter object (list(f) == []), then in Python 3 bool(f) returns True which is why you do not get what you expected.

Third line checks if there are any columns, rows or blocks which are not a set of numbers from 1 to 9.
Second line helps define blocks.
Does this help you? You can ask more specific questions if you are missing something.

Could someone explain the idea of this solution? Thanks!

Aside from the concatenation issue on zip(*board), it returns False when a solution is True..
can't seem to figure out a fix in Python 3

If I remember right, the solution was written in Python 2. In that version of Python zip() returns a list. In Python 3 zip() returns a zip object instead, so you will need to convert it into list in order to concatenate it with lists.
board + [*zip(*board)] + blocks can help resolve this.

This truly is a beautiful solution. I am particularly intrigued by the nested list comprehension, gonna need to write it out as a set of for loops to grok it in fullness, I think, but I would definitely not object to one of you discussing it at length. :)

no (and think about the spoiler flag, plz)