“Deprivation” here is just a verbal formula and the principle could be framed in alternative ways. Being in terrible pain such that you would accept a coin-filp between relief of that pain within your life or death might be characterized either as a deprivation of basic comfort and dignity or as the presence of an active evil. I don’t think it matters much which formulation which might use.
I think the Heuristic is sufficient but not necessary to characterize a life as being on balance suffering. An outcome with a lower probability of death or relief might also be sufficient to so characterize a life.
That said, I think the Heuristic is readily defensible. Wouldn’t things have to be pretty awful in your life for you to accept the flip of a fair coin that delivers relief on one side and death on the other? (I suppose you could have an attitude of icy equanimity toward death, but if you’re like that you could just abandon the heuristic and determine whether your life is, on balance, suffering without the heuristic just using razor-fine utilitarian calculations instead).