He didn't say Sakura would have an entire arc, he said that he would write about Sakura just after Kakashi. That's not the same, it doesn't mean the arc will be center on her.
And do you really think Kishi would have time to write an entire arc for Sakura, while next arc will surely be the last? Next arc will certainly be about Team 7 and its resolution, so it's possible that Kishi will keep his promise and write about Sakura, and resolve her character.
Well, it's not like Kakashi has the entire arc for himself as well. It's about who is getting a big development. So yeah, Kakashi is getting his closure very soon. Sakura is on hold as well as Sasuke with dropping hints of his intentions. Naruto gets bits by bits but that's how a protagonist works.
Btw, thank you for sharing on the book info.