Sadly that is the most plausible part, for two reasons.
For one thing, machines need constant maintenance from qualified professionals, or they break down. And, once they’re broken, you have to pay for even more expensive repairs or buy a new one. Slaves, on the other hand, you can abuse almost to the point of death and they will keep functioning; they’re also somewhat self-replicating.
The second reason is why so many people want to bring slavery back: the cruelty is the point. There’s a certain mindset that really loves lording it over people; and even better if those people are so in their power that they physically/legally cannot escape. See also: people who verbally abuse wait staff.
Of course, it’s up to debate whether a truly alien lifeform would be thinking in such a human way, but it’s certainly possible.
(That being said, these people are lunatics. Douglas Adams once said, ‘Isn’t it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?’ and I think that sums their insane rantings up perfectly.)
I can’t argue against the cruelty part, but I would think that it would be very likely that aliens with the power to travel across interstellar distances and subjugate entire planets would have developed self-repairing technology.
Maybe there is something about gold that we don’t understand yet.
Maybe its seen as preferable and moral to create and later free a new slave species (self sustaining biological robots) then to liter the universe with artificial non-intelligence
I can not recommend actually believing this stuff with no bit of proof but if you keep an open mind its not completely inconceivable.
It’s not completely inconceivable that I put an Invisible Pink Unicorn in your garage, but it’s a similarly silly thing to give credence to considering there are far richer sources of gold in our own solar system which have apparently never been mined.
That and the fact that we basically have an unbroken line of skeletons between us and our chimp-like ancestors which suggest we evolved over time like everything else on this planet, no alien slavers required.
It doesn’t matter if there is something about gold we don’t understand yet, because based on what we do know about everything else, humans evolved naturally and leprechaun aliens don’t exist.