Earlier quoted context omitted.
There's one on the website: http://horizon.io/faq/
IMO it hand waves Meteor away without acknowledging (perhaps out of ignorance) some of it's other strengths. I'll watch Horizon but I'm taking it all with a grain of salt. Standard new javascript framework protocol people.
But Meteor isn't completely prescriptive. There are several pieces that can be swapped out: You can use Angular, React, or Blaze on the client, for instance.
From what I've read, it doesn't look like Horizon is a complete framework, just a database client/server component that can be used with any framework.