Misapplying Subjacency Duplexes To Adpositions Misunderstood As Structure Markers [1]

Martin & Doran (2023: 32-3):

Recognition of subjacency duplexes opens up the possibility of a logical alternative to the experiential analysis of adpositions discussed in Section 2. Examples (2) and (3) are reworked as (19) and (20) below. Here participants in clause structure are analysed as being realised by subjacency duplexes, with progressive (α β#) or regressive (#β α) structures as appropriate; their α is realised by a nominal group and their β by the relevant clitic.

Blogger Comments:

[1] To be clear, subjacency duplexes were said to be motivated by relations within nominal group structure, but here they are applied to a relation between a nominal group and an adposition. 

Importantly, these adpositions are not structure markers, but markers of functions (clause roles); see Matthiessen (1995: 370). Because they are not structure markers, they are irrelevant to the concerns of the paper: interpreting structure markers in terms of subjacency duplexes.

[2] To be clear, applying existing SFL theory, the participants in (19) and (20) are each realised by an adpositional phrase consisting of a nominal group and an adposition, on the model of a prepositional phrase:

Cf Matthiessen (1995: 370):

In summary, Martin & Doran have merely rebranded an adpositional phrase as a subjacency duplex.

Nevertheless, the question is — even ignoring all the misunderstandings involved — what explanatory advantage does the subjacency duplex analysis have over existing SFL theory?