This is true, but not because the continent is peninsular. Western continental shores benefit from the prevailing winds carrying the more moderate air masses of the ocean inland. Water has a higher heat capacity than land, which means it both absorbs and releases heat more slowly. So it doesn't cool off as much or heat up as much as land. On top of that, Europe doesn't have the coastal mountain range that runs down basically the entire west coast of the USA and blocks those oceanic air masses from moving further inland. So the moderate Atlantic climate extends much deeper into Europe than it does in the Americas. For all the romanticizing of terrible Belgian weather in the USA, the hotbed cyclocross regions here have much more severe winter weather on average.