1e-10 is reeeallly close to zero, therefore 1e10 * 1e-10 is also close to zero.

That's what your math sounds like to me.

His math is x ~ 0, hence x / 10 = 0, hence x = NaN.

The starting point is just wrong that Denmark can't play a role when it comes to climate change. Denmark can make a change. It is like saying that when voting that no individual vote or county matters, when the opposite is true: every vote matters in the same way.

Every kg CO2 saved is good... (obviously we should strive for the most economic way to save CO2).

Isn't it true?

    f = lambda x: (1/x) * x
    f(1e309)

yields NaN, not 1.

(So I guess Denmark is at least 1e309-sized in some metric).

No.