This is an excellent article. Too many people believe that kids getting colds will help their immune system get stronger. In reality, getting sick is almost never beneficial.
The information about the hygiene hypothesis and how it is really more about "old friends" is also very important and not widely understood. Many don't know about the hygiene hypothesis at all and those who do usually interpret it incorrectly.
Kids getting colds allows them to develop immunity to some of the hundreds of endemic upper respiratory viruses that constantly circulate throughout the human population. They're going to get infected with many of them sooner or later, so better to get it over with sooner. This protects them later in life when their immune systems are less effective.
I haven't seen any research to support that contention -- rather, everything I've seen suggests you don't get coronaviruses like colds once off, so you don't "get it over with". Instead, they can weaken your immune system and these viruses can stay around for years, as discussed in the article.
The article is a mix of half-truths and speculation with a few random facts sprinkled in. It doesn't represent any sort of scientific consensus. Don't take it too seriously.
Infections with typical rhinoviruses and coronaviruses build adaptive immunity to those viruses (as well as those that are genetically similar). This typically reduces the severity of subsequent infections. They didn't stay around for years.
When we talk about things that "strengthen" or "weaken" the immune system that's really an oversimplification. There's a lot more going on in the complete system.
You say the article isn't fully truthful. Where are your sources to back up your claims? What are your credentials? Or are you just pulling it out of thin air?
The information about the hygiene hypothesis and how it is really more about "old friends" is also very important and not widely understood. Many don't know about the hygiene hypothesis at all and those who do usually interpret it incorrectly.