arXiv replaces any URL in the text of the abstract with a link with text "this http url"; it seems the authors did not know this and just embedded a bare URL in their abstract.
TLDS = "[a-z][a-z]+"
A more restrictive TLD list would have prevented this, but I certainly don't want to be the one to add new TLDs all the time, so I can see why the code looks like it does.