I personally think that despite the two sharing the movie in terms of screen time, it still worked as a Captain America movie nonetheless.
Steve still felt like our main guy because his half of the story was him being proactive and pushing the story forward, and having many plot threads continuing from his previous 2 movies, like his distrust of the government and his friendship with Bucky, while Iron Man was mostly reactionary to Steveās actions, and they even tied his parentsā death to Steveās story, as the Winter Soldier, aka his best friend, was the one who killed his parents. Plus, Steve does have 1 extra minute of screen time over Tony, which, yeah, I know it isnāt saying much, but Iāll take what I can get, I guess.
The movie doesnāt have many plot threads and characters from the Iron Man movies like they did with Cap. I mean, Happy Hogan isnāt in it, Pepper Potts isnāt in it, itās just the parents stuff, which they related to Steveās friend. Sure, War Machine is there, but he did become an Avenger at the end of Avengers 2, so Rhodey is the only Iron Man character in this movie.
The cast that has the most focus in Civil War are all Cap-centric (Steve, Bucky, Sam, Natasha, Sharon, and them addressing Peggy in the movie), plus Iron Man and Black Panther, which they both related to Steveās best friend, as he killed Tonyās parents and TāChalla thought he killed his father, plus Rumlow at the start of the movie before he died, but yeah.
Bucky has more screen time than any Avenger outside of Tony, so itās kinda weird that for an Avengers movie, a non-Avenger character surpasses the rest, as they all were removed from the movie after the airport battle until we see them right before the credits roll.
The movie just has a big focus on Steve and Buckyās friendship, which is fitting for a Cap movie, since the previous two movies focused on the exact same thing and gave it a conclusion, as the mid-credit scene serves as an epilogue to the trilogy, with Steve knowing that Bucky is finally safe and under protection at Wakanda.
Baron Zemo felt like a Cap villain, as he was using the Hydra book (which, btw, Hydra is related to Capās movies) to take control over Bucky, who is his best friend, and we even got some lore on there being other Winter Soldiers out there, making the movie feel like a direct sequel to Cap 1 and Cap 2.
I think calling Cap 3 an Avengers movie is totally valid and makes sense, but I donāt think we should consider people being totally wrong for calling it a Captain America movie, even though he is toe-to-toe with Tony in terms of screen time.
But nonetheless, Steve still felt like the main protagonist of the movie.