By definition, a semiotic system has at least two levels of symbolic abstraction.
By definition, a symbol is something that means something other than itself.
Martin's claim is that because content is paradigmatic and expression is syntagmatic, there is only one stratum. Note, incidentally, that by this logic, it could be alternatively argued that there is only one axis, and therefore no system–structure cycle.
This is like saying all my squares are blue and all my triangles are red, therefore there is only one shape (or only one colour).
If content conflates with paradigm and expression conflates with syntagm, then:
content/paradigm is realised by expression/syntagm.
content is realised by expression
paradigm is realised by syntagm.
There are still 2 strata and 2 axes in this conflation.