They mysteriously didn't mention us, even though we send more than a billion emails a day too! :)
- SendGrid
For the right level of "sponsorship" I am sure they will be willing to do a follow up article.
Considering that mailchimp is one of the big names in the business i don't think this is submarine PR (Although the article is good PR for mailchimp). Do you assume every article about a Google or Microsoft product is submarine PR?
[I am not in anyway affiliated to Mailchimp, and don't even use their services. We use Amazon SES]