The comment you pick up on (and run with, if I may say so...) simply made the point that the two because stretches in the speech [the one a phrase, the other a clause] perform a similar function with regard to the utterance as a whole in providing alternative causes for the event described and this is why they can coordinate despite their different structures (and the different kinds of meanings these structures construe as structures). As far as models are concerned, there is nothing in what I said that posits a stratal rather than realisational relationship between function and structure;
There are several interwoven confusions here.
There is no realisational relation between function and structure.
There is no stratal relation between function and structure.
The stratal relation is one of realisation.
There is a realisational relation between strata; e.g. the lower stratum of content realises the higher stratum of content.
There is a realisational relation between system and structure; this is the relation between the paradigmatic and syntagmatic axes.
There is a realisational relation between function structure at a higher rank and the syntagms (forms) of the lower rank, as when a Process (clause rank) is realised by a verbal group complex (group rank).
What Bartlett said construes two levels of symbolic abstraction on the content plane, two strata related by realisation, even though the technical terms he uses — "stretches of speech" and "the utterance as a whole" — construes them as if in a part-whole relation.
It is this stratal relation that allows us to speak of the same meaning (semantics) being realised by different wordings (lexicogrammar), as when the meaning 'cause' is variously realised as a dependent clause or as a prepositional phrase.