As above, thrulines mean absolutely nothing other than that Ancestry's algorithm has detected possible relationships to a line in your tree, based on information from other user trees. The information in those trees may be wrong and always needs checking thoroughly by doing your own research into the proposed lines before accepting any of it, in my experience. Conversely, other trees may be correct and yours may be wrong. Either way, it all needs to be checked for accuracy, as Ancestry have no way of checking that any proposed thru lines are correct, only that they appear to match a software calculation that has recognised potential relationship patterns between two or more member trees.
I have no thrulines to one of my Great Grandfathers. The reason is that every other member tree on Ancestry in which he appears, wrongly identifies him as a different man with the same name, although born into a different family in the same year and location. Consequently the ancestral line is completely wrong and will never match against my tree through Ancestry's algorithm. How do I know that all the other trees are wrong and mine is correct? Because I have copies of his marriage certificate and the birth certificates for all of his children, which between them contain information that differentiates the two similarly named individuals and makes it certain which is correct. One or several other trees have been constructed without fully researching the two people concerned, and the incorrect lineage has spread through the common practice of people accepting information from other member trees without making any checks!
