It's very simple. The only movies in the MCU that you can theoretically watch without seeing previous films are the first movie for a character. Ant Man, GOTG, Dr. Strange, Black Panther, Spider-Man. There are still plenty of movies that are coming out in the MCU that a newbie could watch without being confused. If the newbie happens to like what they see, they can then go back and watch the older films.
No one should be going to see Captain America 3 as their first MCU film.
Their point is that you can't watch JUST the Captain America movies and understand everything that's going on in 3 because that movie prioritizes being an Avengers movie with elements of Captain America 1 & 2 as the vehicle moving it forward.