@Sea beat me to it, but his argument was that folic acid is non-enzymatically formed as a part of normal folate metabolism because of the oxidative environment in the cell (explained around 15:50).
I'm not a biochemist so I don't know if that's really possible. He presents it as fact, but I can't find any other source to support what he says. Does anyone know?
The intracellular compartments that contain folates are environments that are highly reducing. There is not going to be any significant formation of folic acid by oxidation of dihydrofolic acid which is itself only a very weak antioxidant.
I agree that his first point about "natural" was quite weak. When most people say methylfolate is natural, they don't mean it's derived from food, they mean it's the form that's active in the body. In any case, I'm sure I'm not the only one who would like to know what other errors he made.
Just in the parts I watched, and I'm forgetting some things, but besides attempting to present methylfolate as an artificial substance, he also suggests that a normal role for the enzyme DHFR is to reduce folic acid - a substance that is not naturally present in the human body.
He also claims that taking folic acid and methylfolate will have the same identical effect in the human body. But, as one example, folic acid is an inhibitor of DHFR while methylfolate is not. Thus folic acid will, for example, interfere with the recycling of BH2 to BH4 by DHFR.
He also claims that folic acid works as well as methylfolate, yet the conversion of folic acid to methylfolate requires several steps, one or more of which may not be very efficient in some people. It's true that even people with significant MTHFR defects can normally use folic acid (or better folinic acid) in place of methylfolate and be able to make sufficient methylfolate. The problem, though, is what sort of excess amount of folic acid will be required to do this and, more importantly, what will the effects be of that excess amount in terms of inhibition or stimulation of certain enzymes or genes.