The title of the video is you’ll never improve unless you learn endgames.

My opinion is the title is true once you get to a certain intermediate standard. Beginner players often lose their pieces carelessly and that has to be the first thing to improve surely.

I would definitely say the endgame is far more important than trying to become an expert at the Ruy Lopez, for example. It is called the Spanish Torture for a reason. It is because there are so many variations and not even top players always play it.