2. Because it doesn't reallly matter; the "how did the missile detonate" question is readily answerable, and all that gas in the balloon is well above the impact site and smoke cloud.
The parent poster expressed disbelief because of an entirely incorrect assumption about how missiles work.
Hydrogen is a more common choice than helium for unmanned balloons. Only 12 of 101 weather balloon launch sites operated by the US's national weather service use helium[1], and that's with unusually cheap helium access in the US. I would expect other countries to use helium even less frequently.
> GEN. VANHERCK: Yeah, absolutely. There was a warhead in the missile. You can see that explosion on TV as it goes through the lower part of the balloon and right there through the superstructure.
