What you describe isn't Gladwell's argument. And anyway he his a popularizer of social sciences. A lot of the arguments are actually quotes from various university professors.
The cold winter theory was proposed by Richard Lynn, who is an editor of the Mankind Quarterly which is by many described as a "white supremacist journal."
Psychology Today writes about some of the problems with this theory: https://www.psychologytoday.com/us/blog/unique-everybody-else/201211/cold-winters-and-the-evolution-intelligence
This is fairly well known stuff for anyone who has had an interest in genetic research, racism etc. Angela Saini in her book "Superior: The Return of Race Science," also covers this.
In short I am not making up any of this. I am just letting you know what the mainstream view of this cold winter's theory is.
I hope this has clarified things and we can put this issue to rest. I am not interested in any further discussion as you are not contributing anything of substance.