Additionally, this simulator likely assumes that the warheads will explode with the designed yield. There is a surprising amount of work to rebuild and maintain a nuclear warhead on a regular basis for it to work as designed. If Russia hasn't maintained their warheads, then they might only give fission yields (like Hiroshima), not H bomb yields. At worst, it may not even be fissile and just end up being a dirty bomb.

I'm betting China is doing a better job of maintaining their warheads.