Yea, I agree with that advice. Clear all can bring small performance/battery dings. When you hit the clear all button, all of the Activities are removed from memory. It’s possible that the app’s process can stick around (especially if an app has a service running), but likely the apps will be completely removed from memory. You shouldn’t do this because that’s the operating system’s job. It’s constantly evaluating what you’re doing, what you’ve done recently, what you haven’t done, etc to free up memory and make the things that you do frequently as fast as possible. So, the OS may keep some of your app’s processes cached and ready to go because you use them all the time.
When you forcibly remove those apps that you use often from memory, it means that the next time you open that app, the whole process must be rebuilt from scratch which is a small amount of extra work (leading to slightly worse performance and battery life).
Removing an app from memory could be useful if you want to clear out all of the state on that app. Maybe you’ve navigated down a few screens in the app. Swiping that app away from the recents screen will remove that state and you’ll start over the next time you open it (also helps with apps that are not functioning correctly). Clear all could be useful if you’re running really low on memory and the OS isn’t doing a good job of keeping up. Otherwise, I don’t use the clear all option at all.