I actually first read it in a .pdf file here on the nexus. That does kind of explain things, maybe the author half assed what Shulgin wrote and claimed it as fact.
EDIT: wow, I just noticed that I said "you'd essentially synthesize DMT in your stomach" in my first post, no wonder the hostility, that was a complete idiotic mistake right there, I didn't read that claimed at all.

Self-Synthesis of DMT Derivatives

Tryptamine derivatives and beta-Carbolines have been detected as
endogenous metabolites in mammals, including humans. Methyl transferases
that catalyze the synthesis of tryptamines, including DMT, 5-MeO-DMT and
bufotenine, are found in human lung, brain, cerebrospinal fluid, liver
and heart (McKenna & Towers 1984). In the pineal gland MAO is the primary
inactivation pathway of serotonin, a neurotransmitter synthesized from the
amino acid tryptophan. If MAO is blocked by harmine, harmaline or other MAO
inhibitors serotonin can be converted by the methyltransferase enzymes
HIOMT and INMT into psychedelic tryptamines (serotonin --(HIOMT)-->
5-MeO-trypt. --(2*INMT)--> 5-MeO-DMT).

So, ingesting l-tryptophan to increase serotonin levels, a candy bar to
increase the amount of tryptophan getting to your brain and natural
plant material containing 25-50 mg harmine/harmaline to block MAO, all at the
same time, is supposed to cause your pineal gland to synthesize substantial
amounts of 5-MeO-DMT (Most 1986). This is extremely dangerous for persons
with existing amine imbalance or schizophrenia. For normal, healthy people
possible consequences are bad.

A potent inhibitor of INMT, which is a necessary enzyme for the synthesis
of DMT and 5-MeO-DMT, is found in particularly high concentrations in the
pineal gland. A bypassing or inhibition of the synthesis of this inhibitor
might be responsible for trances and other psychedelic states achieved
"without drugs" (Strassman 1990). See Strassman's article for more info and
speculation about the pineal gland.