Does the article actually mention that parts of Earth are no longer habitable already, or just that the conditions are moving into that direction? The title of this post seems to indicate the latter.
https://en.wikipedia.org/wiki/Planetary_boundaries
It is just a way to quantify something very elusive. The point is we are doing a lot of harm without any consideration for our actions. Profits over all.
Crossing 6 out of 9 planetary boundaries would not necessarily mean that parts of the Earth have become uninhabitable for humans, but it would signify a very high level of risk for abrupt and potentially irreversible environmental changes. These changes could severely disrupt the Earth's systems that make the planet habitable in the first place.
The concept of planetary boundaries is designed to identify the "safe operating space" for humanity. Crossing multiple boundaries increases the risk of triggering non-linear, abrupt environmental changes on a continental to planetary scale. These could include extreme weather events, loss of freshwater resources, collapse of ecosystems, and other severe disruptions that could make life increasingly difficult for humans and other species.