Well... The more radical political philosophies (at any given time) make lots of equivalences (ethical and others) between slavery & other stuff. Wage labour today, and obviously older institutions like tenancy, serfdom, indentured labour, etc. Even many free thinking, but politically mainstream people (eg t cowen) make non-ethical comparisons between slavery and employment.
It's easier to agree with if you widen your concept of slavery, beyond the specific institution of African slavery in the US... during the later years. That system was so brutal and unethical by slavery standards. So, it's hard to compare to.
Before, during & after chattel slavery, the Americas had other systems of "slavery." Most were not that* brutal, but still slavery.
Indentured service was huge, at a certain point. Share croppers. These people probably outnumbered free employees in many times and places.
Serfs were not free to go. Monks were arguably enslaved by abbots. Artisans sometimes bought apprentices as children.
In western Europe, the "tenant" concept that replaced serfdom was often similar to sharecropping.. arguably also a system of slavery.
We know of ancient examples: Greek, Roman, Jewish, Persian, Babylonian... Ancient law codes (including the bible) dedicated a lot of ink to slavery laws. Slaver rights, slave rights, manumission, and such. A Greek slave could be wealthy, and that should help broaden the concept.
Marxism sees slavery in class terms. Ie it's just a name to describe the lowest class, and wage labour is a descendant of slavery.
There are even many modern-economy examples where employers have a say in an employee's marriage or reproductive choices. That arguably is slavery-like.
Anyway... once you widen the concept of slavery with multiple examples that you easily accept as "obviously slavery*, drawing analogies to other institutions is easy.
Indenture, for example, exists, today, in practice, widely... in many forms. Invariably, the "slavers" think it's ethically sound.