While that's true, there are clearly screws or rivets in the video's of it. It's extremely unlikely to be solid metal, though it may be filled with rock/sand/something.
Probably they hit it with a rock and decided it didn't sound hollow. They may also have noted an absence of seams. Seams could be hidden with welding and grinding, but that would suggest it wasn't assembled on site.
"Probably". I wouldn't trust journalists on this sort of thing. It would be completely insane to make this solid. It's almost certainly welded sheets with ground welds, assembled off-site.