That's great advice if you're 18 years old; but some things are worth the risk.
If you're not all in, you're not in at all. While this certainly applies to work in some cases, it applies as much as again if you want to start a family.
Modern relationships are a hell because everyone has their eye on the door at all times - ready to bolt at a moment's notice if something goes wrong, or if something better comes along.
You'll never have a family of your own with that mindset; it's a problem that causes itself.
Seems like black and white thinking. There's a big spectrum between those two extremes, and no good reason you can't healthily occupy a point on that spectrum. In fact, many points on that spectrum are probably a lot more healthy than either extreme.
In both case I think the advice is "do not get invested that much that you loose everything including yourself if it fails".