No.
When performing an index rebuild operation using SORT_IN_TEMPDB = ON or SORT_IN_TEMPDB = OFF (this is the default), both require additional free space in the data file (mdf) to hold the newly rebuilt index before the old one can be dropped.
When using SORT_IN_TEMPDB = ON, you also require additional space in tempdb, the size of the index being rebuilt!
So if you were under the impression that SORT_IN_TEMPDB = ON doesn’t require the additional space for the rebuilt index in the mdf file, that is not the case.
A basic example for say a 1GB index:
| SORT_IN_TEMPDB | Data File Space Required (GB) | Tempdb Space Required (GB) |
| OFF | 2 | 0 |
| ON | 2 | 1 |