I agree that saying gerrymandering affects everyone is sort of off-topic and distracts from discussing the precise impact being discussed, but it’s really not equivalent to “All Lives Matter”.
“Black Lives Matter” => Stop police murdering black people
"All Lives Matter" => La la la, I can’t hear you
"Gerrymandering Denies Incarcerated People Fair Democratic Representation" => We should stop gerrymandering for the sake of prisoners
"Gerrymandering denies everyone fair democratic representation." => We should stop gerrymandering for the sake of everyone
The dinner example assumes only one person didn’t get dinner. If instead everyone went without dinner, wouldn’t it make sense to point out they weren’t the only one affected?