I'm also a moderator on the same reddit forum as Jerry. Number of dead trees isn't really meant to be taken as a measure of success, but more an arbitrary indicator of experience. As in, the longer you've been doing bonsai, the more trees you've probably seen (and perhaps helped) die. It's not the only indicator, and perhaps not the best, but it's something.
It's not something we spend hours agonizing over either - as I said, it's arbitrary. There's probably been more discussion on it in this one thread than there has been in the past five years on this topic. =) The number of trees they own and the number of years of experience are probably far better indicators overall, tbh, and those are factored in as well.
The whole thing is very subjective, but it works wonders because you have some idea of the experience level of the folks providing advice. If two people give different advice, and one person's flair says "Beginner, 2 years, 1 tree" and the others says "Experienced, 25 years, 75 trees", you have a pretty good idea of who is providing the better advice. I've always taken the "dead trees" part of the equation with a grain of salt. Although in the past 20+ years, I have probably killed more trees than most people will own, so it does seem fairly accurate for me. Most of those were in the early days, but I do still lose some from time to time. But sure, not everyone is going to have the same death count for all kinds of reasons. That's why it's just one factor in the equation.