In season 2 episode 10 when they're in the time machine room, Flynn says:
All their owners tried to change history and got caught in temporal loops because you can never ever ever violate causality. The very first time traveller learned that if you change any event in the past, any event of significance in the past, your timeline collapses, your entire history wiped out.
However in season 4 episode 5, at the end they send Darrington Dare back in time. As a result of their meeting, rather than dying the next day, he continued to live for another 62 years. Yet there doesn't seem to be any difference to the current timeline, other than an addition to Darrington's page in the book of librarians.
How come the timeline didn't collapse?