The author is incorrect, or at least misleading. People get colds in Antarctica quite often or they certainly used to:

https://www.cambridge.org/core/journals/epidemiology-and-inf...

Though bear in mind the obvious - people get infected when they allow in outsiders like contractors, wintering staff arriving, tourists, etc. Over winter when you're in isolation there's nowhere for viruses to come from and the infection rate drops. Remote stations would probably also proactively get people to isolate so the whole crew doesn't come down with the lurgy at once.

It's not that your immune system is better, you just have a much smaller social network. Wintering staff are generally in good health anyway (the medical is extensive) which probably helps a bit. You also get a mandatory flu jab (this is not normal in my country unless you're old).

Once you get back into the general population, you pick up all the seasonal colds that you missed.

Edit: also check those references!

An outbreak of common cold at an Antarctic Base after seventeen weeks of complete isolation.

(from the abstract they never found the culprit - perhaps it got frozen onto a surface and then thawed later?)