Yes, because their entire business model is built around email marketing, and despite not using MailChimp then they're as far as I know the biggest player. Give me a couple of hours and then I can guarantee 99.999999% durability for almost no cost through Google/Amazon services. I would be very sad if MailChimp can't gaurantee the same.

Edit since I can't reply: We're talking about durability of their mailing lists, not uptime. At least that's my intention. That Google/Amazon or MailChimp might experience downtime doesn't have any impact on the durability promises. I would also expect MailChimp (rather than all their customers) to do regular backups.