Unfortunately, I do not have access to Klessinger, Szczerbinski, Varley, 2007, the publication mentioned.

I have a suspicion, though, that this research would be much more productive if the researches had deeper understanding of philosophy, especially of mathematics and language, and subjects such as symbolic logic and model theory.

Few years passed since my last studies in this area, and I have practically no neuroscience knowledge, but the simplest bet would be to say that the operator/verb example are just special cases of the relationship between objects.

The bootstrapping process might be explained by the fact that in mathematics we encounter purer form of this mechanism. In case of language, there might be much more distractions, making it harder to muscle this one particular area.