Even if there was lead in the pewter (I thought so too) pouring out molten lead like that wouldn't be much different than just dropping a brick of lead.
Lead contamination comes from distributing it- in a dust or powder, or even just small chunks. Or worse, in the form of lead salts, which are far more readily absorbed into living things.
Pouring lead into a groove like that, and then removing it, would leave behind essentially nil contamination.
I will note, however, that had he tried a metal with a higher melting point, like aluminum or iron, he'd very likely have experienced a steam explosion, where the metal is hot enough that any local moisture flashes to steam instantly. Creating a ball of hot gas that tends to throw wads of still-molten hot metal.
Back when I worked in a wheel foundry, rumor has it that some fellow, after eating his lunch, wadded up the tinfoil and threw it into the hearth. Molten aluminum, aluminum foil, I mean, why not?
The resulting steam explosion from the residual moisture in whatever food was left on the foil, burned three guys with molten aluminum, one of them severely.
This fellow, with his "pewter" (which probably had a melting point around 400F) and the open-top nature of the mold, minimized- but didn't eliminate- the possibility. If I were him, I'd have had a little more protection than just safety glasses.