Ehhh... it is a custom in the syntactical analysis that if either the subject or object is not mentioned and we know that it is generally a neuter singular (which we know here) and if we know that none other noun previously mentioned can be meant but it is something new we have to supply, we usually say (it is an agreement we have made) it is some [id]. That's just a custom in the syntactical analysis. Maybe you're not aware of that.
"Quid" = no.
A structure:
X is Y
X has a different syntactic function than Y
(and you correctly said that X is a predicate there, I made a mistake; Y is a subject)
You can't have "X is Y"
where X and Y have the same syntactic function. That's not how it works.