They are saying that a lot of Devs stick to older/larger file formats.
Imagine a music file. Your first thought is probably MP3, you maybe even have a specific bitrate for it in mind. MP3 hasn't been good since early 2000's. Nowadays for a good Audio codec you would use Opus, and it can give almost equivalent sound of an MP3 file at half the size.
Similarly you can do that with WEBP/WEBM and the incoming AV1/AVIF, instead of using GIF, JPEG, BMP, etc. Large size saving and equivalent quality.
What keeps the other Devs from doing this, is that these older formats became status quo over time. It is very hard to get behemoth corporations to move away from things, hence why some companies still use COBOL or punch card computers.