They posted it because he collapsed and died during extra time at the Argentina Netherlands match the day after he wrote it. Lots of theories and potential explanations for what actually happened as he’s been talking about not feeling well recently, but you can’t really blame people for being suspicious after what happened with SA and Khashoggi.
His brother posted a video saying he believes he was killed, but he was obviously and understandably in a really emotional state. To add to this, his brother is gay and Grant wore a rainbow shirt (as many other prominent journalist have done during this World Cup to protest the Qatari stance on LGBTQ rights) to a match and was told he couldn’t enter the stadium.