"A vs. One" in this context

As well as hoping to encourage children to catch the reading bug, WBD also hope to get reluctant adults hooked on books. So, for the first time, WORLD BOOK DAY will also have an adult focus, with the launch of Quick Reads, ________ selection of short, fast-paced stories.

This is a CAE, Use of English exercise in which students have to put one word. According to the answer key, only the indefinite article a works. However, why not one?

In English the indefinite article (a or an) should be used for a single item, unless you intend to emphasise the number. So for example we say "he wrote a book" when the writing or the book is the focus, but "he only wrote one book" when highlighting the number of books. Also compare "do you want a beer?" (yes or no, the number is not emphasised) vs "do you want one beer or two?" where two numbers are contrasted and a number is expected in reply.

In your example the number of releases is not the focus, so you should use a.

https://www.englishtrackers.com/english-blog/one-vs-a-singularly-confusing/