Right, you need the neuter article to substantivize an infinitive. (Epexegetic infinitives are simply infinitives used in a particular way—epexegetically!—to fill out the sense.) That said, there’s sometimes little practical difference between use with article and use without, e.g. between το ευ πραττειν αγαθον (“doing well is good”) and ευ πραττειν αγαθον (“it’s good to do well”).