Cotton underwear doesn't work that well if your activity varies. Wear enough cotton to be warm while sitting for hours, and you're certain to be sweaty after five minutes of physical activity. And moist cotton underwear makes you cold.

I tend to wear wool long johns from December to March. Decades of experience with cotton, too. Wool is much, much better.

Especially merino wool. That stuff is fantastic.

It's a well known thing among mountain walkers/climbers. Cotton kills.

That's obviously true. If you wear cotton, you need to be prepared to add or remove layers of clothes as needed.

This is a "technically correct" post.

However, the post is suggesting people in comfortable status quos change their behavior - the OP is explaining that yes, it's totally achievable to be comfortably warm in all activities indoors and out, but it costs money.