Cherrybim obviously if there are Rabbonim who do not hold like Rav Feinstein and people legitimately follow those other rabbonim then Rav Feinstein was ‘not’ paskening for ‘everyone’.
Or to put it another way; I believe Rav Gronish will also pasken for anyone who asks him a sheila he will not (to my knowledge) tell someone “You are not chasiddish, so I will not pasken for you”.
Are you saying that Rav Feinstein would eat from all those heschers you mentioned, that Rav Gronish would not eat from?