Did you understand where the 2^64 came from in their explanation btw?
I would have thought it would be (64^10)*16 according to their description of the string.
The YouTube identifiers are actually 64 bit integers encoded using url-safe base64 encoding. Hence the limited number of possible characters for the 11th position.
Edit: Oh because 64^10 * 16 = (2^6)^10 * (2^4)