The argument is that if even just counting through all values of a perfectly energy-efficient 192-bit counter requires the power of an entire star for many years, it's obvious that you'll never have to worry about someone brute-forcing through all keys of something encrypted with 192 bit strength (or better). And even tapping a supernova only gets you 219 bits.

It's about the feasibility of breaking 256-bit encryption keys with a naive brute force attack. The simplest component of a computer that performs this attack would be a 256-bit counter that iterates through all possible key values, however Schneier is saying that according to our current understanding of physics, even if you built the most efficient computer possible and powered it with the energy output of a supernova, it would only have enough energy to iterate through all the values of a 219-bit counter.

To cycle through all 2^219 possible values in a 219-bit counter would take a huge amount of energy.