I doubt there is any real paradox here: cross-diffusion of metabolites is still a sufficient explanation, given all the exponential
feedback we know well.
We did in ancient history, as I recall, discuss whether factors other than actual nutrients might be exchanged; and nowadays
we'd have to add conceivable surface-to-surface interactions in transport mechanisms.
One would like to think of experimental tests to settle more exotic possibilities.
Do you know if anyone has put together a mathematical formation for syntrophic growth?