The study took 30 speed walkers and put half on the Keto Diet. They looked at their bone density before and after embarking on the diet. While they did show that the athletes on the Keto Diet had less bone density after changing their diet the study only followed them for 3 weeks. So, there is no way to know what happens after maintaining a low carb diet over a long period once the athletes became fat adapted.
Then the athletes embarked on three and a half weeks of intense training, while eating mostly fat or mostly carbs. Afterward, the researchers again drew their blood and rechecked the markers of bone health.The New York Times
They found differences. The markers of bone breakdown were higher now among the athletes on the keto diet than at the start of the study, while those indicating bone formation and overall metabolism were lower. These same markers were generally unchanged in the high-carb athletes. The athletes on the ketogenic diet, in other words, showed signs of impaired bone health.