Yes, I agree there's no direct proof of a secondary alkaloid.  What is certain is that there's something in there which is either biologically active, or acts in some physical way to increase the effect of the DMT (I have to say, this latter possiblity doesn't seem likely based on the available information... I could see where the oil might protect DMT from degradation, but it shouldn't increase the potency that much.... after all, the oil adds weight to the sample).  All scenarios that make sense to me involve a chemical besides DMT which is biologically active.

As burnt points out, it's entirely possible this could be a tannin.  We don't really know at this point.  But the issue of jurema's oral activity, and the isolation of yuremamine still lead me to lean toward this being an alkaloid.  It would be very interesting to know exactly what yuremamine degrades into under what conditions.