This sounds like it could be rather dangerous, no? Connecting up a bunch of old lithium cells of unknown origin and using them hard. I'd also worry about an insurance claim being denied if it burned my house down. However, I very much approve of the DIY ingenuity and repurposing of old stuff.
Yes, this is extremely dangerous. I'm sure some of the people mentioned have the expertise to pull it off, but for most this is like building a massive bomb in your garage. There's a reason that 18650s aren't generally sold directly to consumers, in most applications they need dedicated circuitry to be used safely. And that's for brand new cells, it's another story entirely to be using random collections of "old" cel…
They are, but IME the sourcing on some of those (especially through Amazon.com, which comingles items from different sources) is hinky. That 18650s can be dangerous if misused or if counterfeit isn't an argument for restricting availability, it's an argument for making them available directly to consumers from legitimate authentic sources.