As this also lists lines of code, it sounds more like sources plus packages. Think space that a full mirror (src + generic + arch specific packages) would need.

Indeed, this is the amount of space that a Debian mirror would need to host all Trixie packages. So it's the compressed packages total size, not the space it would take to have all packages installed simultaneously (which also happens to be impossible, because of package conflicts/alternatives and Debian supporting 7+ different architectures).

The 403GB figure represents the total size of all source packages in the Debian archive, not the disk space required for a typical installation which is usually under 10GB for a desktop system.