For supernovae to have had a meaningful effect on the natural baseline incident of grassfires, there would have to have been a dozen or more a year for tens of thousands of years. Or to put it another way, cosmologists should stick to cosmology.

Grass evolved to cope with regular fire whereas the trees they were competing with didn’t do so well; hence the spread of savanna and hence our ancestors spending more time as bipeds. There was already enough in the environment to enable grass to evolve in this way; a supernova every few tens of thousands of years would have had zero evolutionary influence.