Tens of millions people are in literal slavery today, whether sex slaves, child labor or other forced labor. Reading about what they endure, it seems disingenuous at best to claim slavery and employment are "clearly not that different" by reducing it to the nature of the compulsion behind it. It's like claiming a bullet and a tennis ball clearly aren't that different; they both involve projectile motion through the air under some degree of momentum, though the nature of that momentum can be different.

For things to be "not that different" implies they are interchangeable to an extent. I'd certainly be interested to know the result if you surveyed a random sample of 100 employed people and asked if they'd be willing to forego work to enter slavery.