I guess it depends how deep you dive... It is true that on the surface, git gives you the user access to full blobs only and calculates the difference every time you access blobs. But when you go in packfiles the content is actually diff'ed because it compresses so well.

In the context of the discussion, since we're interested in the on-disk format, it's more accurate to say that git will try to diff binary blobs, fail at that and so store the full content of blobs.