
(This seems a bit silly in the case of a KRKR end-game where it is obvious that none of the engines is going to blunder away his rook, no matter how little time it has left. But you have to draw a clear line somewhere. Otherwise things will deteriorate on a continuous scale, because there is always an intervention that is just a little bit worse, and you would indeed end up with people "coaching" their engines by telling them where to attack, vetoing moves that are strategically bad in a way they know the engine cannot see (e.g. trapped pieces), etc.)
As for the book: I don't really believe in books at all. Opening theory never interested me much, even when I still played as a Human. So Joker simply plays without a book, thinking up its own moves from move one. So I am 100% sure Joker can never 'get' a bad line, he will play only what he likes. And by sufficiently randomizing moves, it is not easy for opponents to 'book' Joker in advance. I might still force the first move by randomly picking from a table, to get somewhat more variability. One could call that a 1-move book.
I guess you were indeed unlucky then, if a line looks good at 10 sec, but bad at 1 min.