they based it off of what happened to hyuga. tsukasa was positive hyuga had died 7 years ago during the battle. but they were still able to revive him now. another thing is they hypothesize kaseki's situation. kaseki's been revived twice and after each time he mentions that his body pain had disappeared. personally i think that there's still a lot to question about it tho. like maybe hyuga was just at the BRINK of death so it was still possible to save him through petrification.
I can see what you mean with the brink of death, but when Senku's neck was completely broken he wasn't breathing anything. Now with that stayed I think that you need to be turned to stone right after or right before you die because your brain is still active for a few minutes after you die. But as Senku stayed we don't know if turning to stone will fight off cell death, which courses aging, I don't think it does but we'll find out. ╮(￣▽￣)╭
thats what i thought as well, "brink of death."
Can someone explain the whole immortality part. Does that mean if a bullet was to go through their head they wouldn't die? how about those guys in the earlier chapter who died from inhaling sulfur gases during the time of Senku vs tsukasa. or is this immortality just stops them from aging?