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Sure, moderate-to-vigorous physical activity is correlated with better bone mass, especially in men. It's less of a correlation in women, though. The study we are commenting on was in women.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6197664/

We identified that the positive effect of MVPA relative to the other behaviors on bone mass is the strongest overall effect in men. Furthermore, women might decrease bone fracture risk through PA increase and SB reduction, despite the fact that no clear benefits of PA for bone mass were found.

So the correlation is not so clear, and differs between men and women. However, the single biggest correlation to hip fractures is fall frequency, which makes sense. If you fall more, you are more likely to break a bone. The high B6/B12 group had significantly more falls than the low B6/B12 groups. More physical activity can put you in more situations that might cause a fall. So while the physical activity itself might help with bone mass, you can't just look at that in isolation. Increases in physical activity in the elderly is correlated with higher fall frequency.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5131688/

In this study of free-living older men, the associations between PA levels and subsequent risk of falls depended on mobility status. Among men without mobility problems (who were a more active group), there was an incremental increase in number of falls (IRR, 1.5) for every 30-min increase in MVPA. There was a steeper increase in number of falls at the highest step counts (≥9000 steps per day, a level achieved by only the most active 10% of the study sample), but lower step counts were not related to falls risk. Conversely, among men with mobility limitations, number of falls increased incrementally with lower step counts and lower levels of MVPA, and a nonlinear association between falls risk and SB was observed; SB >10 h·d−1 was associated with steeper increases in number of falls.

So if you don't have mobility problems, more physical activity is correlated with higher instances of falling. I am not a researcher of falls in the elderly, though. I was just looking at the data and seeing a lot of confounding variables. If you look at the speculation by the authors, it seems to be mostly an issue with B6 and post-menopausal women with low BMIs.

A possible explanation for the interaction between total vitamin B6 intake and BMI is not clear. At low BMI, fracture risk was higher, and a larger number of incident hip fractures thus occurred, yielding higher statistical power. We speculate that the possible mechanism of excessive vitamin B6 exposure increasing fall risk through neurological symptoms could particularly aggravate fracture risk in women with low BMI, who are more prone to fracturing their hip when experiencing a fall. However, adjustment for falls had little influence on the estimates, and the association between falls and fracture risk was not altered by adjustment for vitamins B6 and B12 intakes. It also could be that the possible interaction between vitamin B6 and the steroid receptor might be most influential in lean women, who have a reduced capacity for production of adipose-derived estrogens.

So perhaps the B6 is causing/exacerbating neurological symptoms in frail older women, which causes them to fall more. The other speculation is that B6 is affecting their estrogen levels in a way that affects bone strength.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1096/fasebj.8.3.8143940

recent novel results indicate that moderate variations in the intracellular concentration of pyridoxal 5'-phosphate (PLP), the biologically active form of vitamin B6, can have pronounced modulatory effects on steroid-induced gene expression. Specifically, elevation of intracellular PLP levels leads to decreased transcriptional responses to glucocorticoid, progesterone, androgen, or estrogen hormones. Conversely, cells in a vitamin B6-deficient state exhibit enhanced responsiveness to steroid hormones. One aspect of the mechanism by which these transcriptional modulatory effects of PLP occur has recently been shown to involve interruption of functional interactions between steroid hormone receptors and the nuclear transcription factor NF1.

So B6 deficiencies seem to correspond to enhanced responsiveness to steroid hormones. Perhaps elderly women that have low B6 are reacting stronger to the estrogen in their bodies, which affects bone strength. Elderly women with low BMI that have higher B6 levels might be reacting less strongly to the estrogen in their systems, which is already low at that age. It's an interesting mechanism I was not aware of. Perhaps saying the study was worthless was premature. It is interesting that B6 deficiencies actually correspond to higher sensitivity to steroid hormones! I'd love to see a study like this that looked at men. It seems in this case it is mostly elderly women with low BMI that are affected by higher levels of B6.