You may be absolutely correct and it would still be legal slavery. This is also generally not labour for the community in the general sense - there is an entit

You may be absolutely correct and it would still be legal slavery. This is also generally not labour for the community in the general sense - there is an entity drawing a profit from prison labour (the state if it’s a publicly owned prison, or the prison owners if it’s privately owned). What you describe would be community service, which you may be able to argue is not slavery (and I would say is also an acceptable form of punishment for some crimes if it reduces recidivism).