That's a different kind though and only mentioned on the side in the linked blog. If you don't know what you're doing and you jump into freezing water for the first time, chances are that you'll breathe in water. Happened to me as well. Let me tell you, it really sucks. If you're unlucky, this can definitely be enough to make you drown. But with training you can easily overcome this problem. I have jumped into arctic waters many times since then with zero issues. What the blog claims, is that there is a vague, general physiological risk from some kind of mixed response targeting your heart. I have to say I've never heard of this before (except for people with pre-existing conditions) and the blog does a poor of explaining the biology beyond high school level anecdotes. It also cites zero sources. So while there is a risk, it is not what you might think after reading OP's link.