How Short URL of 6 letters prevent guessing short URL?

According to the requirements, shortened links should not be guessable.
But if we use 6 letters of base64 for a short URL , it spans a space of around 69G possibilities, while the system is designed for working 30G URLS.
This means that around half of combination of 6 letters are potentiality a valid tiny URL, that means it is very easy to guess a valid tiny URL.
So what I miss here?

My guess is this is not a strong requirement. There’s little harm in being able to guess a URL that is public anyway. And, private URLs are restricted via permissions as discussed in chapter 12.

It is essentially a trade-off between usability (ie. a user should be able to easily share the link, maybe even memorize it) vs. security/privacy (ie. it should not be easy to find working URLs).