Adam Clarke on 2 Peter 2:18

They speak great swelling words of vanity - The word ὑπερογκα signifies things of great magnitude, grand, superb, sublime; it sometimes signifies inflated, tumid, bombastic. These false teachers spoke of great and high things, and no doubt promised their disciples the greatest privileges, as they themselves pretended to a high degree of illumination; but they were all false and vain, though they tickled the fancy and excited the desires of the flesh; and indeed this appears to have been their object. And hence some think that the impure sect of the Nicolaitans is meant. See the preface.

Those that were clean escaped - Those who, through hearing the doctrines of the Gospel, had been converted, were perverted by those false teachers.