FIFA have said that those numbers are in line with what would be expected. They’re sick!
I don't see where FIFA is saying that. In the original article that is linked to, it's the Qataris who are making that statement. Also, how do we know the numbers are not what would normally occur in the same population? No other figures have been given for comparison, so we have no real context.
Now, I think there's little doubt that the conditions for many, many of the foreign workers in Qatar are exploitative and abusive.
However, as the Guardian article this is based on makes clear, this is deaths among migrant workers from South Asia in all sectors of the economy in Qatar, including those in office jobs and those working in areas that have nothing to do with the World Cup facilities, over a ten year period.
What it doesn't say, for instance, is what the total number of such workers in Qatar is. (A quick Google search shows that there are over 1.5 million of them).
I'm not sure what the expected yearly fatality rate for 1.5 million South Asian workers is, so I don't know whether 6,500 deaths over ten years is exceptional or not.
It's perhaps with noting that the same Guardian article that gives the 6,500 total figure also states that there are officially only:
37 deaths among workers directly linked to construction of World Cup stadiums.