It's accurate, though. When I am evaluating devices to buy, a metric I care about is "after I buy this, how long will it remain up-to-date with security patches?" And the answer to that question is "on the day that you buy it, it is already several months behind on security patches and will not improve." That metric is not the be-all-end-all of support, but is meaningful, and low or negative values have the correct interpretation in that context.
well in that case many cheap android phones/tablets would have negative support periods, considering they don't release any updates at all.