They were ordered by the FTC in 1997 specifically not to make that claim:

“ shall not represent, in any manner, directly or by implication, that:

5. such product is more bioavailable, more absorbable, or more effectively utilized by the body than other forms of calcium, or is superior to or more effective than other forms of calcium in the prevention or treatment of bone ailments,”

Details can be found here:
https://www.ftc.gov/sites/default/files/documents/cases/1997/10/d9267.do_.htm