Ancestry knows who your DNA matches are. Thrulines looks through all the user trees that are marked as searchable, for possible routes that could link you and your match. The results are simply hints for you to review and research to either confirm or discard.
I had numerous Thrulines hints to cousins on one branch of my tree that were, until recently, all correctly shown. A recent tree has appeared which incorrectly links my GGM to the wrong husband for some, but not all of my GGM's children, and all of those cousins are now shown in Thrulines as half cousins, despite up to 8 correct trees being referenced by Ancestry if I click the Evaluate links, Thrulines is choosing to display the results from the incorrect tree. It is a hint tool, nothing more. Beware and treat it accordingly.