The Rambam is referring to both points.
You can acquire a slave- i.e. someone who is already a slave- by purchasing them etc.
Or can you can acquire a slave- i.e. a free non-jew who is becoming a slave- by purchasing them etc.
The term "acquiring" refers to both aspects.
FWIW, I heard from a dayan that this is still relevant nowadays.
There's a big ...