That's the often repeated argument. But as a counter point you have the Google Pixel. Google owns the hardware, even the SoC, and the software. And yet, battery is still poorer than third party Android phone manufacturers. And let's not even compare to Apple.
So controlling the entire stack isn't enough. There has to be a desire to do better, as well as technical competency.
third-party manufacturers cheat for battery life benchmarks and they don't even get that much better battery life. the current state of the pixel phones allows me all day battery life without such headaches.
in my experience with OnePlus, it aggressively kills and/or throttles applications that you want to be running in the background (e.g. fitness trackers or even audio players) even when you put them on the battery allowlist.
at the same time, my iPad just randomly decides to drain all of its battery even when it's been sitting unused without any apps installed. I don't have this problem on Pixel devices. (you do get this problem as soon as you install some misbehaving apps but that's something you've done to yourself)
So controlling the entire stack isn't enough. There has to be a desire to do better, as well as technical competency.