While I agree that it would certainly be ideal if a speed limiter could account for the context that the car is in, you’ve missed a lot in drawing your conclusion that it would be useless without being able to do that.
Hitting a pedestrian is not the only type of accident. If you rear end a car going 25 mph at 70mph it is not a guaranteed death sentence for all. Especially if the driver brakes, which some do not, but some will. And this is ignoring cases where there isn’t a tremendous mismatch in speed. Like, even if it reduced residential deaths by 0% but it reduced overall deaths looking at all situations, it would be a net gain with literally nothing lost. We are looking at the aggregate here. So, it isn’t relevant if you think of one specific situation where you believe 70mph isn’t better than 90mph or whatever number.
Reaction time and braking distance are affected by speed. In some cases, the person going 70 might be able to slow down enough to have the collision be non-fatal. Reaction time goes down and braking distance goes up as speed increases. If a speed limiter gives just enough time to occasionally make an accident non-fatal, then in the aggregate you have fewer fatal accidents.
In fact, taking braking distance into account, I don’t think you can even say that over the millions of miles driven, that a speed maxed at 70mph isn’t going to, occasionally, lead to a situation in a residential area where someone was able to just get out of the way in time because the car covered 30% less distance between the time the pedestrian reacted and the time the car reached that spot (or an even larger difference if the driver noticed and braked at some point as well). But again, it doesn’t matter if it’s few to none in this specific scenario, because a speed limiter of 70 will certainly reduce fatalities overall.