Learned at this point it's best just to not engage with the "is Smash a fighting game" argument. People are so set in their ways about that for whatever reason.
On a more general note though I find it odd how exclusive some people think the fighting game genre ought to be. Platformers and RPGs exist in all sorts of forms, but nobody is dying on the hill that Kingdom Hearts isn't an RPG because it's not turn based or something. Expecting fighting games to stay regulated exclusively to their traditional roots is reductive in a way that no other genre is expected to be.
How are arena fighters not fighting games, let alone platform fighters?
Traditional Fighters were made to facilitate the Competitive nature of 1v1 into the game. Part of the reason SF2 was such an hit was because it was one of the only games where you could play against another Person at the time, and typically the games people consider Traditional (Both 2D, 3D, and also Anime) tend to be balanced around this aspect. The balance is definitely not perfect all the time, but the effort is there in how Characters and systems interact with each other
Arena Fighters Typically tend to be Fanservice games, since alot of the Arena Fighters are based around Anime I.Ps like Dragon Ball, Naruto, My Hero Academia, etc. These games try to put in as much Content and characters as possible, even sacrificing Individual Depth amount the roster (Both in Homogenizing the roster and even the ways to use attacks and such) and sake of Balance if need be. There still can be a Competitive nature to them of course, but I think it can be superseded by the priority to represent the Series the Arena Fighter is based on to the best of it's ability, at all costs
For Platform Fighters, I think that's mainly because Smash has had such a Monopoly on the genre for so long, and honestly still does to this day, and Smash's whole thing is to Appeal to many sides of the coins, with the main focus on Casual Aspect in many ways to play the game, with lots of characters, and ability to have Chaotic Party Esque gameplay with Items and Stage hazards. Even though there is a Focus to have Competitive elements in the game, because it's not strictly built around it might be why there's kind of a disconnect.
For as many Different systems there may be in types of Platformers, and the fact that Action RPGs like Kingdom Hearts use Action gameplay instead of Turn-Based, alot of aspects thats focused in Regular Turn Based RPGs (Stats, Big explorable world, Captivating Story, Different ways to approach the game, etc) are focused in Kingdom Hearts and other Action RPGs.
Now I do think Arena Fighters and Platform Fighters are Fighting games, and even among people that might not consider those games Fighting games, that doesn't mean they find them un-enjoyable. I think the main reason other people might not consider those types of game "Real Fighting games" is because there's little to no options in those genres that have the Same focus on the Competitive aspects that build the Formula of Traditional Fighting games, at least ones that are popular.