It's a stretch for two reasons:
1. The answer choice just says Anaphase 2, no more than that - you generally don't want to reach beyond what's presented in an answer choice (e.g. but what if it's anaphase II with nondisjunction?). In the context of the question, you're assuming each of the answer choices is occurring under normal circumstances. It's very hard to select correct answer choices when you start extending hypotheticals because that can be a slippery slope - for example, Prophase I is a true statement answer choice to this question because of crossing over, but what if something went wrong and no crossing over occurred? Try not to extend your reasoning beyond what's given unless you are explicitly required to do so (like in application style questions).
2. Anaphase 2 (the separation of sister chromatids) still wouldn't be responsible for causing diversity. As the Further Details section points out, anaphase 2 just separates the chromatids - the genetic diversity was generated in prophase I with the crossing over (and metaphase/anaphase I with independent assortment that creates a wide variety of possible chromosome combinations, which itself is a form of diversity). Chromatids failing to separate doesn't really increase genetic diversity, even if it technically is another possible outcome.
