I think in this case the reason DigitalOcean isn’t mentioned is that its only product offering is a VPS. But the list notes
All Dedicated/VPS hosting supports installation. This is not a link directory page.
This is to say that VPS providers don’t have to do anything in order for Let’s Encrypt to be used with their products (customers can inherently obtain and install their own certificate on a VPS or dedicated server, and don’t need the provider’s permission or assistance).
Whereas for shared web hosting, customers do need the provider’s help or permission, and so this list focuses on distinguishing between shared web hosts that do or don’t facilitate the use of Let’s Encrypt certificates.