It's a 58-character password. Every password strength checker I try certifies it as extremely strong. Remember, it wasn't cracked, it was published in a book by a Guardian journalist.
And what Assange wrote to the journalist was only a part of the password. Assange never wrote the second part. Apparently the journalist had to combine both the said and the written parts to use it but published the whole combination in a book.
How could Assange use such terrible opsec as to use only obscurity and no random key in a own-rolled password generator?