You got it. Copies of your gated posts are kept on the our end which will run access-control and serve it to authorized users. You get to keep all of your content and you won't ever be locked to Subscriber Only.
For authentication, the bare minimum of the "OAuth 2.0 Authorization Code Grant" [0] is being used. A blog post detailing this process is on the roadmap.
You can login in as many browsers as you like. This will raise some eyebrows but, right now, I'm adopting a sort-of Netflix's "Love is sharing a password" motif. This might change later.
On feed readers, the content of the post will just be a message saying you need to access the website to read the post. I wish there was a better way of handling this use-case but haven't found one yet.