Casual is a very subjective word in reality.
to some it’s about how easy it is.
to me, “casual” would be appropriate for eternal , not because of the difficulty mind you, but in term of playtime. (though of course some could simply not care about seasons )
someone that plays a lot might have enough time to make new characters every seasons
while someone that play casually (i.e : 1h a day or less) might prefer eternal.
tldr : for some “casual” is about lack of “skills” whereas to others it’s about how much time they can invest in something.
personally i describe myself as “casual” ; sure i killed uber lilith etc… in S1 , but i still played merely 7h / week roughly.
which , to me , meant i played rather casually even though yes , in term of “skills” i’am okay i’d say considering i play usually rather hard games and in the diablo franchise itself i did about everything ; i even did the holy grail SSF in D2.
but still ; i’d say i’am playing rather casually at D4 since i don’t have much playtime to spare. (but i still enjoy it though )