I not only believe it now, I've believed it for years. I don't think this is a new concept. The first time I read about it, the issue was children. And it was found that exposing kids to dirt, microbes, bacteria etc helped build their immune systems. Furthermore, depriving them of that exposure increased the odds that they'd develop allergies.
Let's take it a step further. Your body, my body, is host to a vast universe of critters, a microbiome. Our health depends upon them. They come, the go, our microbiome changes depending upon our environment.
Now, we're aboard a spaceship, traveling to a distant star. The ship is either close to being sterile, or it's a time capsule of the microbial environment that existed when the doors were closed and off it went into space. The people onboard are divorced from Earth's evolving microbial environment.
What happens to their health, years into their journey? The human species has never been completely separated from Earth's microbes. We have an interdependent relationship. Will the space travelers fall ill? Will they experience unexpected problems, unexpected because we've never done what they're doing?
In my opinion, solving this problem is right there at the top of the list of things which must be solved before humans can safely travel the enormous distances of space.