I don't think the parent is saying there's not value in that wait list. I think he's saying that Dropbox probably valued the team, Mailbox's standalone potential, and potential synergies more than just the "marketing list".
Both really. Whatever monetary value you would put on that marketing list is tiny compared to the reported sum. Give me a few hundred grand and I'll get you that good of a mailing list. Dropbox doesn't need to spend 10s of millions on that.