Ah ok. Crossed streams.
The instant death rule in the players guide is something that is already part of 5e. The difference with the (optional) rule, is that it unties the chances of dying instantly from a direct correlation with the amount of hp’s you have.
This has the effect of actually making you much less prone to instant death in the beginning levels, but then it flattens out a little. There’s still the level bonus increase, so you do have that increasing greater ability to thwart death (becoming more heroic), but not at the same rate as the PHB.
Again this strikes me (excuse the pun) as a more gritty but still heroic ruling. There’s a balance in there, with lower levels getting a big boost, then a gradual increase in survivability, which reflects an S&S with a pinch more grit, but still modelling heroic progression...just not at the same level as standard 5e which quickly sweeps up into super heroic.
Again really easy to ignore that rule (though instant death will be much more likely at low levels).
Personally I’m fine with it and will use it in my games. Very happy to have that optional suggestion as it fits with the balance of grit (not too much) and heroism in the S&S I’d like to play. I want hero’s, but not super high fantasy hero’s.
These optional rules makes the game accessible to me. Everyone is a winner.