Bone remodels in response to load. That is not a controversial claim. It is the mechanostat theory, and it holds up well in the literature.
The real question for a Pilates studio is how you deliver meaningful load without breaking the person who came to you because everything else hurt.
Our answer is a progression. Reformer, Tower, and Chair work to establish control. Konnector-loaded standing work to build tolerance. Then coached free-weight strength. Bone density improvements typically show up on DEXA between month 9 and month 14.