Don't know who "they" are, but that's just one possibility I'm sure will be considered.
As far as it being "impossible" for the reversers to deploy in-flight, yeah, they're not *supposed* to, but Murphy's Law and the Lauda crash show that it can indeed occur, even with the air/ground safety logic. In fact, check out this link for the Lauda report:
They mention 3 different possible scenarios that caused the reverser to deploy, and were unable to determine which one caused the crash. According to one of the appendices, it appears that the main problem was with Pratt 4000-series engines, and not so much the JT9D, RB211, and CF6 engines on the 767 fleet.
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