Let me try to explain it another way. There are currently two ways of integrating an ESP which are being discussed here:
I have been speaking to Sparkpost and Elastic Email about possible integration. Both show interest to offer their services but are unfamiliar with XenForo code and see no examples. Their service API's are superior according to them.
The thing here is probably that they want to integrate their native API
without SwiftMailer. At least that would be my guess. This is the first way to integrate an ESP. Just plain application (XF) <-> API (ESP) communication. That's why they would call their APIs superior. Because they simply have more functionality inbuilt.
The second way is to use a SwiftMailer integration, like for example Sim is using that one for SparkPost, I peeked into the SendGrid one etc, which means there is already a lot there to begin with (but not everything). This means application (XF) <-> Bridge (SwiftMailer + Integration) <-> API (ESP).
What these integrations do is to simply make the ESP's native API compatible
with SwiftMailer. What the ESP devs now would have to do is to integrate their stuff into XF using the already existing libs (or extending them) following the already existing XF code for handling various parts of emailing.
Otherwise, if they really want to create a native XF addon with no external dependencies other than those of their APIs, then you're right, the XF documentation on that one is weak and a common interface would make sense.
But then again, there is already the option to use and extend SwiftMailer + integrations. So either way, the ESP devs have to decide on what they want to get familiar with - either XF & SwiftMailer or XF & Guzzle.