This is a common misconception with git. Yes, conceptually git will treat each commit as complete tree, but the actual pack files are storing deltas because anything else is way too inefficient. Loose objects are only the initial format stored before there is enough data for pack files to make sense.
The concept is what's important, here, since they were correcting the idea that git works mainly on diffs, which it doesn't. The diffs are merely a storage optimisation.
