The title is a bit of an exaggeration... Maybe I'm missing something but there's actually nothing in the article about the FFMPEG setting used, most importantly

The title is a bit of an exaggeration... Maybe I'm missing something but there's actually nothing in the article about the FFMPEG setting used, most importantly the encoding bitrate or quality. There's also no quantitative comparison between the old video and the new. It's relatively straightforward to measure the noise added during the encoding process. That being said, H265 definitely does improve filesizes.