I never read severe. Just burns.
Even first degree burns (e.g., the equivalent of sunburns) can take a few days to heal. The types of burns that would be associated with bomb-making or detonating—and certainly the type of burns that would have gotten him admitted to the hospital—would likely be at least second-degree, and could take one to three weeks to heal. Unless they were very minor, it’s unlikely the guy was burned on Monday and then took a picture on Thursday (at the latest - that’s when it was posted) with no sign of burns.
And, again, if he had burns on his hands, he would not also have an IV in his hands.