it seems that a third referendum has been ruled out.

I think that is a bit premature. Let's see what happens after around Dec 12th. I agree May is trying to rule it out, but equally there are amendments being proposed that as Laura K puts it:

The BBC's Laura Kuenssberg says it supports what "some in Number 10 suspect - that is vote falls, Parliament essentially takes over from the executive".

And if that happens, any options could arise, whether Mrs May agrees or not.