The following reference found on the citations page of the PDF you posted, lutesium, might shed some light as to the nature of the acid catalyzed dimerization of ring substituted propenyl benzenes -- particularly, isosafrole.
Tricyclic dimers of propenylphenyl ethers-I NMR and stereochemistry.
MacMillan, J; I.L.; Morris, D.J.
Tetrahedron 1969, 25, 905-914.
Rhodium, do you have online access to this journal article? It might prove interesting to read.
BTW, lutesium, the paper might come in handy if one was to ever determine that such HIGHLY substituted phenethylamines are indeed active, but, for now, all one can do is guess.