Consider {lo spesi'u poi crino}. My interpretation is that it refers to people that form a couple (or couples), and additionaly we have the information that these people are green, which presumably is important to understand which couple we're talking about. It has the same possible referents as {lo crino poi spesi'u} and {zo'e poi spesi'u gi'e crino}. The relative clause can only limit the possible referents of the description, not expand it.
To talk about a green part of some spesi'u, we could say {lo me lo spesi'u me'u poi crino}, or more simply {lo crino pagbu be lo spesi'u}, or, in particular, {lo speni poi crino}.
But, using the convention from the text, we would read it another way. {lo spesi'u} would still, of course, refer to a couple, but then the relative clause would act on the reference of {lo spesi'u} to extract a part of it that is green, even if it does not form a couple any more. The relative clause can, then, create new possibilities of reference. It is still restrictive in the sense that it takes a reference and then restricts it to a part of it.
Only now could I come up with this last interpretation. It surely wasn't intuitive to me. I will reflect on its consequences. But for now, what do you think?