There are certain topics that are so settled, that to engage with anyone with a contrary view is tantamount to giving them credibility and a platform to reach the uninformed. Some topics simply don't have "both sides" in any meaningful sense. Flat eartherism, Holocaust denial, and antivaxxerism are ones that immediately come to mind. Anyone who denies the mainstream consensus on those subjects is either a moron, a dishonest person with selfish or malicious motives, or both.
Denial of human-caused climate change is close to being in that bucket by this point (the next 2 decades will determine the truth of it). From the perspective of those advocating for action against climate change, deniers have blocked any sort of meaningful action for nearly 30 years. Their actions have led to unprecedented, potential economic, humanitarian, and ecological crises.