Researchers report they have engineered an enzyme that can convert 90% of that same plastic back to its pristine starting materials.
There is a difference between wh-relative clauses (1) and that-relative clauses (2). The first are formed by wh-movement, whilst the second are a result of a complementizer merged in the head of C:
(1) They said [CP that [TP they know [CP who [C ∅ [TP found the solution]]]]].
(2) They said [CP that [TP they know the key [CP [C that [solves the problem]]]]].
The wh-part in (141) (from Radford's book) is an adjunct. The wh-adjunct which is extracted from an A'-position will land in an A'-position (intermediate Spec,CP). This is how relative clauses such as (1) and (141) are formed.
However, (2) is formed by merging that in the head of C. There's no movement involved. Notice that in (1), the subject of the embedded verb find is who. who in this case is a subject-wh element. who may act as a subject and an object in other sentences. that cannot because it's a head. Head do not act subjects or objects.
Notice further that in (1), the wh-subject acts as a subject of the embedded TP who found the solution. In (2), the subject of this embedded clause (i.e., solves the problem) is the moved DP the key. Based on this, that in your sentence is a head not a specifier.
Researchers report [CP [TP they have engineered an enzyme [CP [C that [TP can convert 90% of that same plastic back to its pristine starting materials]]]]].
Do not take these labels for granted, the CP that can convert 90% etc. is an adjunct modifier of the NP enzyme. CP modifiers do no necessarily contain subjects.