Author here, I had the same question and looked into it. The author of that comment seems to be onto something because hygrine is indeed found in nightshades as well as in coca. Interesting.
The first reference on that page is "The role of hygrine in the biosynthesis of cuscohygrine and hyoscyamine"
Cuscohygrine does occur in those plants yet it's precursor hygrine does not. How it then gets there without us being able to detect hygrine could be because it only occurs in very small concentrations or is produced and then quickly and wholly converted in to its cusco metabolite, or that it's produced through a different biosynthetic pathway.