This is not actual slavery, but it is atrocious working conditions and very serious human rights violations. The conditions may approach slave-like conditions, so there's good reason to use the word, but quotation marks are probably accurate. This is journalism, after all, not propaganda. (But, you definitely make a good point.)
Particularly, The Guardian has not made this distinction in the past, search for "site:theguardian.com sex slaves"
(Further note that one of the methods of coercion being used in Qatar, taking the persons documentation and putting them in an immigration bind, is famous for being used in the sex slave industry)
> Actual slavery is when human beings are legally owned by others.
He's not saying that sex slaves aren't by definition slaves; he's saying that legal ownership is irrelevant, because sex slaves are, in fact, slaves.
To be slightly punnish, this is not an academic exercise.
Slavery has other forms, and being held in captivity for reasons other than legal punishment and forced to work generally is considered one of them.
This comes very close to a proprietary relationship, and it's even partly supported by the legal system which helps to keep the workers/slaves in their miserable "employment" relationship.