As someone who's an all-weather cycler, this is not true. You don't get nowhere as wet walking, especially with an umbrella. Also practically everyone owns a raincoat, but nobody owns a bike poncho. If the rain is heavy, visibility at bike speeds is also a significant issue. I once faceplanted because I literally couldn't see a grey bollard in cataclysmic rain.

In case you do that, also bring a fresh pair of socks. Rain has the nasty habit of sneaking into your shoes while dribbling down and there's nothing worse than going the whole day in wet feet.

How are the lemmings with glasses accomplishing this? I thought ice and snow would be what makes me switch back to a car for the year, but at this rate it's going to be light rain blinding me.

Just look out for those leaves.
When wet they can be as slippery as ice.