Not sure about “officially supported”.
Honestly, at this particular point, I can’t recommend building any integrations for Evernote in Drafts. We state as much in the integration guide. They have, without notice dropped support for several of their methods of integration, and made no clear public statements about the future of the API that exists. The Evernote action step only continues to work because we are maintaining our own fork of their iOS SDK which they abandoned and no longer support.
The functionality continues to be there for compatibility and to support existing users on iOS, but not sure how long it will continue to function.
So, yes, the example To-Do action we provide could be modified to support nested lists, but I wouldn’t rely on it because it may stop functioning on any given day depending on Evernote’s changes.