I'm not sure it's helpful to call this actual slavery. Actual slavery is when human beings are legally owned by others.
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.)
The distinction is purely academic, and worthless for real-world use. Sex slaves in America or the UK are not legally owned, but they are nevertheless slaves as far as anybody sane-minded is concerned. You would not dream of referring to them as "Sex 'slaves'".
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)
I disagree with your assumption that sex slaves aren't by definition 'slaves' because they aren't legally owned. They are bought and sold (from what I've learned at the movies) so to me they are actual slaves.
Note also that his emphasis was on legally, not on "owned": It's not legal to buy and sell people, therefore one cannot legally own slaves. The fact that people exchange money for them, and treat them as property, does not make it legal, but their treatment definitely makes it slavery.
Chattel slavery requires ownership and is a specific form of slavery which was common in the US. Its manifestation requires slaves to be treated as property by the legal system.
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.
It might very well be actual slavery. The "recruiters" are probably not employees of the company that commissions the labor, instead they are selling these men to companies who then "rent" them out.
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.
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.)