That's a talk about brainwallets, which is something else. Brainwallets are generally chosen by humans; you can't rigorously prove how much entropy they have, and in practice it's usually much less than people realize.

With a proper 12-word phrase, each is randomly chosen from a list of 2048 words for 11 bits of entropy per word. That's 132 bits of entropy, which is not crackable. (With a 24-word phrase you subtract 8 bits for a checksum; I'm not sure about the 12-word format, but 124 bits of entropy isn't crackable either.)