Yes I do suppose that's true, but I don't think that makes it any less proprietary in nature. The same could be said of many mail providers such as Gmail, Outlook.com, etc. They have their own way of doing it that, externally, looks like mail. Internally it's something altogether different.
This is proven by sending an email from one Gmail account to another. It is all but instant. I swear once the email arrived before the Enter key had debounced! SMTP can't move things quite that fast.
I don’t think it matters how it’s implemented internally, just like the end user doesn’t have to care about which language the IRC/web/Jabber server is implemented in. As long as it speaks the correct protocol to the outside it doesn’t matter and that’s the beauty of standardized protocols.
Right which is kind of my point. They say they've reinvented email or whatever, but haven't they all? The difference is that this provider forces you to use their client.
The "reinvention" is primarily in the client (but also in the client service) and has to do with getting you to use email differently by having you emails behave in a non-traditional manner.
This is proven by sending an email from one Gmail account to another. It is all but instant. I swear once the email arrived before the Enter key had debounced! SMTP can't move things quite that fast.