No they aren't.
Any interstellar method is a fortiori a terrestrial method as well.
Three is more than sufficient to undermine your proof of concept "not enough fissiles" declaration.
In what respect? The Tsar Bomba gives us an estimate on what the third state buys one, let's be generous roughly an order of magnitude (6mt to 60mt). The analysis I linked concluded that the existing nuclear stockpile was at least 100x too small to power an Orion, so a 10x increase is useful but not enough (and where's the plutonium for 65k stage threes coming from?). If we go with the link's world-wide estimate of 300,000 years of production and blindly apply the 10x estimate, that still leaves us 30,000 years short.
"Bomb on the planet's surface." was exactly the problem specification.
And where does the Orion vehicle hit?
(Would an ICBM be excluded as an answer because it enters space?)
Any interstellar method is a fortiori a terrestrial method as well.
It's a task with entirely different challenges and to which entirely different tactics are optimal. It is just plain not the case that all interstellar methods are a fortiori terrestrial methods - since the very method your declaration of impossibility assumes is sufficiently optimal to rule out any possibility is one that doesn't work when launched from the surface of the target planet.
...The Tsar Bomba gives us an estimate on what the third state buys one, let's be generous roughly an o
http://neo.jpl.nasa.gov/risk/2012da14.html
http://rt.com/news/paint-asteroid-earth-nasa-767/
Seems like a good opportunity to bring up existential risks. And A friendly reminder that NASA is in fact pretty damned important.
Thoughts?