The strange thing for me is why a multi billion company uses MailChimp instead of their own email server?!
It's not strange at all. Email handling is not as you might think.
Most companies, even large ones are NOT managing their own email servers. The times for that have longed passed.
Ensuring deliverability is no longer feasible unless you're a significant company in the field and have a foot in the door so you can call the Google mail team boss on their phone directly and say "Hey John, our mails are now blocked by Gmail, could you fix that for me please". I just made this up but you'll understand.
It's a waste of time and money to do it unless this is your core business and you're a significant mail provider.
Note, we have been managing our own emails since early 2000's. Currently we use a mix of our own servers + recognized cloud providers for the aforementioned reasons. Our own MX servers are used when receiving email is priority (not deliverability) such as inbound sales email; as cloud-based like Gmail, MXRoute and whatnot (all of them, basically) still block a LOT of valid emails. (Edit: hence the case for using one of these when deliverability is priority)
For cases where outbound direction is more important (such as transactional emails for example) we use external cloud mail providers, some of the recognized ones.