Salamun Alaikum.
'nafathati fil-uqad' (113:4) literally translates to 'the blowers in the knots'.
In the context of the Surah, it is clearly a reference to dark occult arts whose existence is implied by the Quran in other parts of the Quran too. These arts and the like are a certain reality from the Quranic perspective and the theme of the surah seems to suggest that the only refuge from such malicious arts is with God alone (a'udu birabbi - I seek refuge in the Lord).
In my personal opinion, practice of such dark arts has never ceased throughout human existence. Its prevalence throughout the ages may have changed at different stages. However, certain societies with particular cultural and theological inclinations along with certain demographics may have always been more susceptible to practice such arts.
Please see the following article which albeit confirms the presence of occult arts, vindicates Harut and Marut of any wrongdoing.
http://quransmessage.com/articles/harut-marut%20FM3.htmI hope that helps, God willing.
Joseph.