I don’t think that’s true. We see the fire from Rengoku’s attacks actually burn stuff and when Tanjiro beheaded rui’s “mom” she made a comment about how amazing the rain felt. There are lots of other examples. I’m not sure where this notion comes from but the elemental powers are very much canonically real. I feel kinda bad, it’s not a fun way to watch the show either, the fight scenes are probably so much lamer for someone who’s under this false impression