From the article you linked :

>There have been 37 deaths among workers directly linked to construction of World Cup stadiums, of which 34 are classified as “non-work related” by the event’s organising committee.

And regarding the 6500 figure (for the 10 year period 2010-2020) :

>While death records are not categorised by occupation or place of work, it is likely many workers who have died were employed on these World Cup infrastructure projects,

The recent heatwave hitting Europe is said to have cause 1000 excess deaths in a week in France alone (again numbers are projections).