Having a reference dictionary is a type of compression.

You need a program that decompresses no matter what, and the ability to reference existing files barely changes it. It's not a post-processing stage where files get copied around. It's the ability to reference arbitrary chunks of data inter-file just like it can reference arbitrary chunks of data intra-file.

At the most basic level it's like taking a compressed file and chopping it in half. The user already has the first half, then they download the second half and "resume" decompressing.

What's your definition or compression? A .tar.gz will de-duplicate files that will be re-duplicated on decompression.