When did he have his back fused? He had a diskectomy in 2014.
Red Wings captain Zetterberg has back surgery
According to Holland in 2018 "Nothing can be done, no back surgery, no rehab, no more time off is going to have any positive impact. Obviously if he plays professional hockey, it's going to accelerate the degeneration and if he does get a bad hit or something, then he's risking a significant back surgery. Henrik has decided that his quality of life is more important than taking the risk of back surgery."
So I've found nothing about having a back fusion. He may end up with one in the future, but it hasn't happened yet.
He played in all 82 games in all three seasons from 2016-2018. He logged more minutes total those 3 years than anyone else on the Wings. In 2018 he logged more minutes than everyone but Dylan Larkin. 21st among all forwards in the NHL in 2018.
So excuse us if we're a bit skeptical of his total inability to play.
At the same time, I don't think anybody doubts the severity of his back injury and can completely understand him not wanting to continue and it would have been utterly ridiculous for the league to not allow him to be on LTIR with those documented back problems.
However this all was true when he was playing in every single game the previous 3 seasons. He was a warrior and battled through, perhaps foolishly even. He could've tried to do it again, but at some point enough is enough. Nobody could blame him for hanging them up.