On my system /usr/share/dict/words has ~45k words. That's 15 bits of entropy per word, significantly more than XKCD (which assumes 11 bits).

So I'd agree that there probably aren't many unique dictionaries. The strength of this password scheme is that it works even assuming the attacker knows your dictionary.

In practice, I guess that an attacker will use a concatenation of all popular dictionaries. This will make things slightly harder for them (e.g. if they have 50k words vs the 30k in your dictionary) but not by a huge factor.