They have never directly said it's not on rails. They have said over many interviews that
1. current space combat was just a place holder for launch
2. the SSSP is something completely new and expends space combat in major way
3. the SSSP is a completely new pixel space
4. that the current space combat has nothing to do with the new SSSP
taking in all of those statements it's clear that it would be foolish to assume that it IS on rails.
now how big it is or what it entails who knows.
All they have ever said definately about the SSSP is that it is a "completely new pixel space", which could mean literally anything or nothing.
They've never said it's not on rails (they just said the new rails missions weren't the SSSP) in fact they've said nothing other than it's a "completely new pixel space".
I mean personally I think they'd be mad not
to be aiming at something that at least equals Jump to Lightspeed in scope (as that is what people want and largely expect), but that doesn't mean there are
doing that at all.