The issue is that becoming a Boeing whisteblower is a 1-way gate: once you are one, you're a member of that group for the rest of your life.
Which means if you have a lot of whistleblowers, because you've got a lot of problems, then you have this ever-expanding pool of people who are "Boeing whistleblowers". Which means the probability a Boeing whistleblower dies in any given year goes up as the pool expands.
Which is fine and normal, unless the narrative question we're answering is: "is Boeing killing whistleblowers to deter future whistleblowers?"
And one conclusion is: "no, but they're being deterred anyway, because everyone is insisting they were intentionally killed". Basically the "chilling effect" is the conspiracy theory, not any actual action.
Though I would note a better way to see if they were doing it would be to look at the death rate of current Boeing employees, since if you were killing whistleblowers that's what it would presumably look like - people presently employed by Boeing dying under explainable circumstances without ever taking any action.