i dont think sans has 1hp
i dont particularly believe that, in universe, sans has 1hp.
he might have 1hp in the code, but there is a genuine difference between a character's coded stats and their in universe stats. using papyrus as an example, in the genocide run, his def is coded to be iirc around -20000 which allows for you to kill him in one hit when he spares you, despite the fact in game it says its 3, which shows that their coded stats and in universe stats are not the inherently same, and alongside the toriel betrayal kill, it sets a precedent that killing a monster when they are sparing you allows for you to kill them with just one hit, no matter their hp or your LV.
now, in the genocide run, you don't kill sans when he's sparing you. but you do kill him when his guard is down as he fell asleep, similarly to how papyrus and toriel's guards where down, whether on purpose (with papyrus) or through a belief you are a harmless child (toriel), when they spare you. Also, I should mention, you're LV 20, ATK 99 and do 9999999 damage to sans when you hit him, I think that would kill any normal monster, or human, to be fair.
another thing towards my theory is that when flowey grabs the main 6 monsters in the true pacifist ending, hes hurting them. you see that they're all clearly in pain, just based on their expressions alone, and if flowey is strong enough to keep undyne and asgore trapped, then its safe to assume he wouldn't find it hard to take out sans' believed 1hp.
he is the weakest enemy, which could point towards sans having 1hp, but its only specified that he can do one damage, which compared to weaker enemies like whimsun, who can do 4 damage, it would still make him the weakest damage wise even if his HP was higher.
any grammar or spelling mistakes r bc i havent reread this b4 i post it