That is simply untrue. Decarboxylation also occurs at room temperature, but the reaction takes place at a much lower rate. Any tested sample of dried cannabis flower will contain the cannabinoid acids and smaller amounts of the decarboxylated cannabinoids. For a ready proof that you can find and choose yourself, look at the lab test results of any online hemp store—any cannabis flower invariably contains decarboxylated cannabinoids (because that's how physics works... relatively unstable chemical groups are prone to decomposition).