That's sort of like saying Facebook scaling is a "non-issue" because it's just a bunch of small, independent profile pages. Of course some elements can and should be sharded, but that doesn't mean that scaling is trivial, especially as you get into more advanced and social features and a rapidly increasing percentage of traffic is logged in and fully dynamic.
I think we agree that "scaling" small (sub-RAM-size) amounts of data to a largely logged-out and cached audience is easy, but I think you think of WordPress.com as much smaller and more static than it actually is. My apologies if I'm misunderstanding your point of view.
If you want to see standalone sites in the top 100 running WP besides wordpress.com, check out time.com, umbrellanews.com, celebuzz.com, and large sections of nytimes.com, cnn.com, and people.com. If you were to spider the top million Alexa domains, you'd find about 17% of them on WP:
Except that as I pointed out, facebook isn't independent pages. Wordpress.com is tons of tiny, entirely independent sites. That's like claiming godaddy is a top 10 website because they host tons of little PHP sites. Tons of little sites is not the same as one big site.
I realize lots of people run wordpress. That doesn't support the claim that it easily scales to the traffic demands of a top 10 site.
It is entirely correct. First of all, they do not serve hundreds of thousands in one install. Second, it doesn't matter. The point is they are individual sites, they are not tied to each other in any way. So lets say they host 100 sites on server X. Now one of those sites explodes in popularity and the server can't handle it all. They can simply move the site to its own server (or a server shared with a smaller number of other sites). It doesn't matter that the data is currently in the same database, as the data is in no way tied to the data of the other sites in that database. So it can be trivially exported and moved to a different server.