Why doesn't the string match the root?
In the text, they seem to suggest that there is a way of quoting characters so that they can be used as terminals even if they have the visible form as a non-terminal. One common quoting form is quotes ('B') which is illustrated.

Character classes ([bB]) are another quoting form. The B in that character class cannot be a non-terminal because it is precisely a character. So the non-terminal B, although it looks very similar, is distinct, and the Bs in the strings are characters, necessary by the definition of string they are using.