I think he just means that, while many Git users may indeed be familiar with blobs, trees, commits, packs, refs, remote refs, and so on, most Mercurial users just know that they have changesets. Most Mercurial users do not know what filelogs are, how they work, what the manifest is, and so on.

That said, I find the original claim, that users of snapshot systems have to know the internals, a bit idiotic. Subversion users definitely didn't, nor did most CVS users. Git seems to just be a very loud exception. Whether this is by necessity due to Git's UI, or whether it's simply that Git's data structures are more visible and discoverable, is a debate I'm not touching. (I'd note that there's a direct 1:1 correspondence for all core Git and Mercurial internal structures, so it's definitely not inherent in the snapshot model. There are even very nearly 1:1 analogs to older DVCSes, like BitKeeper and Monotone. Git users just tend to know more about the internals of their tooling.)

Probably not but he is right - all the git tutorials inevitably lead to explaining how git works internally. It is considered necessary knowledge for "grokking" git.

Mercurial tutorials aren't like that. It is a lot simpler to use so Mercurial tutorials tend to just tell you how to use it and don't mention the internel implementation details.