@Christopher You're right, but when people use the phrase "Marvel movie", they're almost always referring to Marvel Studios, and the fact of the matter - from ALL viewpoints - is that Spider-Man Homecoming does not belong to Marvel Studios.
Not from "ALL viewpoints," just from the bizarrely narrow, business-oriented viewpoint you're using. Obviously it's part of the Marvel Cinematic Universe in story terms. Tony Stark and Happy Hogan are in it. Captain America and the Avengers are heavily referenced in it. The storyline is a direct outgrowth of the aftermath of the Chitauri invasion in The Avengers, as well as a direct sequel to the Spider-Man portions of Captain America: Civil War. There are pictures of Abraham Erskine and Bruce Banner on the walls of Peter's school. Hell, the denouement even features a potentially life-changing event for Tony Stark. From a story standpoint, it is entirely of a piece with the MCU. That makes it a Marvel movie in the ways that matter to the audience, even though it's a Sony movie in the ways that matter to lawyers, accountants, and investors.