You people need to RTFR, seriously.
It was not a "fire warning", it was a smoke alarm triggered by humidity. They don't say wether it was in the cargo or the cabin toilets. I would assume from the reaction of the crew that it came from the cargo hold.
Any half-witted pilot would have the presence of mind to rule out a faulty smoke alarm in a toilet, so it sounds like it might have been something more difficult to confirm like a cargo hold smoke warning. In which case, the diversion is justified but the evacuation might have been a bit of an overreaction.
And a passenger later dying in the hospital constitutes more than minor injuries in my opinion.
