Dy
You have that backwards. You mean like eating in sukkah on shemini atzeres clearly zilzul of yom tov no less than arba minim would be.
Sure people claim that eating in sukah isnt nikur that it is chol, which is of course nonsense (though perhaps it was once true) since there is no way to explain eating in sukah other than the chashah that it is hoshana rabah, I have never seen anyone eat in a sukah for any other reason. (Though again perhaps at one point it wasnt unusal, since many say that it isnt nikur that doing it for sukkos, something which is clearly not true today)
In fact it runs counter to the classic tur that the reason we commemorate sukkos in the fall is precisly when people are going indoors and it is no longer “normal” to eat outdoors as the weather gets cooler, that we leave the house for the sukah. SO how then can one be mezalzel yom tov of shemini ateres by calling it (in practice which is seemingly worse than word) “safek sevii shel pesach” ?
I am sure that the many gedolim who eat in a sukah on shemini atzeres have a valid reaon. PErhaps it is an example of minhag oker halacha