That is true, but remember: the ultimate goal is to block ads BEFORE they even have a chance to load and this is what DNS does. Ad blocking extensions block the request near the finish line, DNS blocks it at start.
When DNS blocks something, that means less work for your ad blocking extension. And less work for your ad blocking extension, means less CPU usage which translates to extended battery life and better performance for device.
If you were to use only uBlock Origin on your mobile device, you'd notice battery draining more than if you used ad blocking DNS and uBlock Origin together.
This is why you should block everything you can though DNS and what DNS can't solve, pass to ad blocking extension.