Is the failure to denounce an abomination—which in this case is cursing—after saying "the most important thing" or "the first thing" and then moving on to another topic, considered a form of denial?

If cursing is forbidden, then the one who curses must be advised, prohibited from committing this evil, and taught the religious ruling. This is based on the Almighty's saying: "And let there be [arising] from among you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those are the successful." And the Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." We hope that you will not be held accountable for what you uttered if it was due to forgetfulness or error, based on the Almighty's saying: "Our Lord, do not impose blame upon us if we have forgotten or erred."