this explanation makes me think TOTP is just adding 6 digits to your password to make brute forcing harder, not really providing a second factor at all!
I think the distinction is that it's two separate passphrases. It's not a matter of brute forcing an n+6 length passphrase (n being the length of your passphrase), but brute forcing a passphrase of length n, then brute forcing a 6 digit number. Both have to happen within 30 seconds. I'm not sure on the actual math to compare the complexities of these two, though. :]