This is the last kata in a series, and, as such, it makes you use all the things you learned in the series. Part of that is converting from a String into an IntStream and back again (a common task in Java streaming). There's no "essentially" about the programmer writing their own flatMap function; that's a specifically defined part of the kata.

Further, it's essentially a translation of another kata of the Dragon's Curve (http://www.codewars.com/kata/dragons-curve), but forces the use of Java streams. As such, it keeps to the contract of that kata as much as possible, which is also heavily reliant on String manipulation and String out.

Finally, a stylistic complaint is a "Suggestion", not an "Issue".