That was not a hard hit either. People watched and it has been replayed many times.
It wouldn’t have been a hard hit if he’d taken the force of the collision lower in his body, as is the norm. It reminds me, somewhat, of the collision that nearly paralyzed former Bill Kevin Everett in 2007. That wasn’t a particularly hard hit, either, actually, but the injury was caused by how he took that hit.
In this case, it looks from the replays like Hamlin tried to let the other player fly over him a bit so he could tackle him with his arms and body, but instead, the player hit him in the chest and chin with his helmet and shoulder. That’s more clear in slow motion, and so in that case, his cardiac arrest could realistically be caused by that collision. NFL collision - helmet and shoulder to chest and chin. Head snaps back as it happens.
From another post of mine: There are a lot of similarities between the injuries of Hamlin and former Bill Kevin Everett. Both had an opposing playing coming right at them that they were attempting to stop, and both took the impact of their collision to the upper body and head region, with the players’ helmets colliding in Everett’s case.
Damar Hamlin injury:
https://www.youtube.com/watch?v=05zP3B23hSE
Kevin Everett injury, 2007:
https://www.youtube.com/watch?v=5_ix6vtuehQ