Finland has thousands of winter swimmers, just normal people, and Russia at least 10x more. The article makes it sound almost superhuman, whereas in real life it's something that thousands of old ladies do daily. No need to travel to the ends of the world and bring a film crew with you.
I mean it could be both. I could imagine that people can become conditioned to it in such a way that they are functional in cold water if they do it on a regular basis and/or were brought up with it.
But I've jumped in glacial lakes before (not on the regular) and it would become life threatening for me very quickly. It's quite scary.
Old ladies don't do it daily. The old ladies that do it often did not started old and gradually increased exposure from 1min once a week to longer swims. An old lady that started as old lady swimming daily for multiple minutes at once would be a dead old lady.
Also, general recommendation is to start from cold showers daily for 2 years and be under 50 years old. Then proceed slowly from there.
The whole thing does things with your body. Also, going to cold water after sauna is something completely different then cold swimming without being completely overheated prior. Hypothermia is an actual real thing that exists and so are heart problems. When you are exposing yourself to cold water untrained, the threshold is not that high for these to occur.
My mom is an old lady, does it daily, and picked up the habit in her 60s. She is very much not a superhuman. Her swim group is all the other old ladies in their 60s in the neighborhood. The "gradual" introduction is to start swimming a few short laps daily in the summer, and don't stop when the weather cools. They use a pump to keep the sea from icing over near the peer; the ice can be about a meter thick nearby.
The freezing water daily is really not recommended if you did not started years ago. It is quite popular around here, I do it too and I have already seen ambulances going. It is easy to overdo it. It is all fun and endorphins until it is not.
The age plays a role in terms of more likely heart problems. Not sure how about the adaptation, how it is affected by age. Muscles matter.
Young males end in ambulances too. The emphasis on old ladies here us because frankly old ladies are supposed to be least capable species. I know they can be tough.
But daily freezing water swim without prior training is bad idea. The processes in your body are supposed to run changed for multiple days and if you do it daily it is not getting recovery. Just like with weight lifting every day, beginners risk injury for doing so.
Also, there is no need to put gradual in scare quotes. The gradual as in continuing from summer is the recommended way on top of those daily showers and listening to your body.
Respect to Finnish and Russian winter swimmers but I think most are swimming in fresh water that is still a few degrees above freezing (under the ice), not sub-zero ocean water.
I don't really think regular winter swimming (https://en.wikipedia.org/wiki/Winter_swimming for the ones of you that are unfamiliar with it) can be compared to swimming 1 km. The former is quite common, but people rarely go for longer swims in water that cold.
