GMAT Sentence Correction (SC) | Re: Some anthropologists believe that the genetic homogeneity

ccooley : I was wondering if you could help me with the following doubts?
1) I am having a hard time understanding how A break the parallelism and why is 'that' required to maintain it. I am referring to OG explanation here.
2) How would you define grammatically what role is dash playing here? I researched use of dash as per grammar rules but didnt find any other example similar to this question.
Thanks