While your code is a valid alternative way to implement @haberman's description, the feature is actually much more flexible.
The "subsequent logic" has full access to the interpolation results and strings. Not only can it escape the results, it can do whatever it wants to them. It can also do whatever it wants to the strings, and then combine everything in any way it likes - it's not even necessary that the final result is a string.
The other PEP example shows generating HTML attributes from a passed-in dictionary. HTML has a number of places where this is helpful, if you have original data.
The "subsequent logic" has full access to the interpolation results and strings. Not only can it escape the results, it can do whatever it wants to them. It can also do whatever it wants to the strings, and then combine everything in any way it likes - it's not even necessary that the final result is a string.