Have you read through their case? It's pretty weak in my opinion. They seem to think that any war with a high number of casualties and insufficient humanitarian aid counts as genocide. By their standard the US committed "genocide" against Japan in WW2, arguably Germany too.
The person isn’t saying that Germany committed genocide during WW2, which is obviously true, but pointing out that by the above definition of genocide, the US committed genocide against Germany and Japan during WW2.