Not sure that's accurate or really tested much in the wild. Let's accept the S21 has been out just days now.
below is link to a video of the S21 shooting video, which would dispute what you've just claimed. I'm sure as time and more units are in the hands of more people, you'll see loads of people saying their S21 has the same issues ALL the higher-end phones have. And that's because of the smaller lens and the amount of light going into that smaller lens.
I will say from what I've seen the S21 and iPhone 12 Pro are quite similar in capability, which is a good thing. People invested in Android, should be able to enjoy a great camera on their phone. Just like people invested in iOS should as well.
There are loads of comparison articles out now. In them, the iPhone 12 Pro and Samsung S21 both took top spot for some photos, with each winning some and each losing some. The differences however were in my opinion minimal and subjective.
Here's the video --> https://youtu.be/8l-92aty8Ps