1) "More than half of the lead currently used comes from recycling." (from your article) - Once the spaceship is built and has left Earth, the lead is essentially gone (for the time being), so the more of those we build, the fewer lead we will have to recycle, and it will get even more difficult to build additional ones.

2) 400 tons for a small capsule, and consider they're proposing a spaceship in which multiple people could live in autonomously, i.e. they need places to live in, but also room to grow food, process their waste, etc. So I'd guess it's much higher than you seem to have assumed.

Oh and btw, I'm fairly certain we also need much of that lead on earth - hence why we're producing so much of it in the first place, so it's not like we have some spare lead in the order of magnitude of, say, 10000s (I'm really just guessing here, though) of tons lying around collecting dust.