Oh, well... I am going to think on this. Ferrous sulfate is what is typically added to foods. It is available for treatment for anemia in that form.

Here is a link to an MRI study showing ferrous sulfate showing up in MRI study
http://www.scmr.org/caseoftheweek/2011/2580.html

And BTW he is using my plastic coated magnets. Notice he had TWO stacked to do this....esp on the water demonstration. Those are neodymium rare earth magnets....very very strong. Not regular magnets commonly available.

Who is that guy....his credentials?

I don't see proof that the black substance was Iron filings. It could be an iron compound, that is just insoluble in water.
Iron taken orally for example as ferrous sulfate is not well absorbed unless taken with Vit C or other acidic substance.

People also ingest iron that is given off cast iron cookware.
I'd like to see that demonstration done with vinegar to simulate the stomach's acidity (vinegar is less acid than stomach acid BTW).

What I see in that demonstration is proof that the iron was added to the
product (so there is no fraud in that way).
Notice that he did NOT test DRY granules of the cereal. This is probably because the iron from the ferrous sulfate, precipitated out in the aqueous environment. It might be very different if he used a pH of stomach contents instead!

BTW Iron is one of the nutrients that is not absorbed when people use acid blocking drugs. Iron, calcium, magnesium, folate, zinc, B12 all require acidic environment to be absorbed. People taking high dose ferrous sulfate for anemia often develop dark stools, as much of that iron is not absorbed at all. The body has strict ways of controlling iron absorption to prevent iron overload. That iron, in nonacidic stools is dark too, like what precipitated out of the cereal mixed with non acidic water.