That phrase is a nominal phrase (meaning, it is used as a noun). That is, "using threats" as a phrase is being used in the place of a noun. In English, a verb can be "nominalized" (made into a noun) by using its gerundive form. So "using threats" is in the gerundive form (a noun), not in the present progressive form (a verb).
Think of it this way. We can say "He has gone back to (the use of threats)." As you can see, "the use of threats" is a nominal phrase. Similarly, in "he has gone back to (using threats)", "using threats" as a phrase is a nominal phrase.
Once a verb is in its gerundive form, it can replace nouns. Ex: "(Singing) is one of my favorite activities" or "I prefer (watching movies) over (going to Britney Spears concerts)."