I assumed the same thing, but I performed a site command search while they were prospects, and they had 1 result present with the explanation of "A description for this result is not available because of this site's robots.txt – learn more"
They uploaded an xml sitemap before I could tell them to remove the robots.txt. and 1 week later, the entire site is now in the index.
I have used the robots.txt to properly block websites, it usually takes 2-3 for all results to drop out the index, so I don't know how that could explain it either.