The streamers were likely conductive Mylar.

While this is potentially true, it doesn't need to be something conductive like mylar to do this. There are very few true "isolators" on the world.

Almost everything is classified as a conductor, or a "semi conductor", because it's such a good insulator. When lightning hits a tree, the wood becomes the path for current, even though if you asked someone if wood was conductor, they would say no. Wood isn't a conductor at like 600v, but touch wood with a 240,000v power wire, and it's absolutely a conductor, for a second, until it catches on fire.

Human skin is no different, you could touch a 12v wire that has 5000amps running through it, and you won't get shocked, because 12v isn't enough to break through the resistance of your skin.

So yes, the streamers could be mylar, but paper ones would have had nearly the exact same result.