That's not really it at all. Genome knew how the Anti-Spirals fought from experience. He knew that they choose a form with the exact same power level and then impose victory by winning.
He must have gotten to a point in his battle with the Anti-Spirals and then retreated. He said himself that he used to be a Spiral warrior even though in the beginning of the series he was maintaining the status quo on Earth so that the Anti-Spirals wouldn't come.
Genome did not believe in Simon's ability to defeat 0% odds until Kittan was able to attain the power for Simon to transform his ship into Chouginga.
Thus, Genome did not believe in Simon earlier when the two were battling in ep 15 as he states in his battle with Simon that the one who came to oppose him before didn't understand the destruction that Spiral power could bring (i.e. destruction from Anti-Spirals...).
Basically, beating Genome is not a 0% probability event as you seem to be asserting despite the vast difference. Transforming his ship to Chouginga was though.