It's one theory among many. The flip side is that autoimmune diseases are often triggered by pathogens (MS from EBV, diabetes from HEV, and more). Air travel and urbanization help pathogens spread. My understanding is that it's not yet clear which are beneficial and which are harmful, but there's ongoing research.

I think everything is a balance.

If you as a human have been exposed to an extremely "clean" environment, your immune system will not have had the opportunity to get trained and strong. So you might get a severe infection at the first opportunity.

However, the other extreme can be very bad. I wouldn't recommend living in a lush tropical forest without any protection from novel viruses and bacteria spread from all kinds of insects and other vectors. Your body may not be able to take that kind of bio-warfare against it.

So, the million dollar question is: How much should we train our immune system by exposing ourselves to the natural environment and how much should we protect it?

Different people will put themselves on different sides of this spectrum.

The "middle" ground is of of course the best, some self-exposure to natural environment and some self-protection. But it is difficult to quantify this "middle" ground and that is why we have many of these arguments.