It is very unlikely that it was a “continuous” compression anyway. Continuous archives disallow random file access property, and games require exactly that for assets. You can’t decompress few gb on average to fetch a sprite of a cat.

The reason games (and software in general) do full downloads instead of binary patches is purely overdefensive and/or stupid. Store software could just check checksums after a patch and re-download only if they fail.

it's a UE4 game so the code is available on Github, I believe it was a header, with an encrypted index into a large binary blob of compressed data.

Well, whether they use full versions or diffs is also going to depend on how much they care about bandwidth.