Don't swipe away shit

BTW, swiping away apps from recent tasks frequently is not a good practice, since it reduces the efficiency of process cache mechanism in Android, thus impact the performance of your device.

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Swiping away apps from recent tasks kills the process of those apps, thus prevent them from being cached in memory. When you launch them later, it takes longer time and much more CPU cycles to create the process and re-initialize the app runtime.
If you don't do that, it generally saves your time and battery, though not so much.

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Most parts are correct. Clearing recent tasks does free much memory, at the expense of later performance and battery consumption for launching those apps again. So if you have a device with 2G RAM, it gains no benefits in practice.

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If you heavily depend on the recent tasks list for frequent task switching, then you may prefer to swipe away the unwanted tasks to make task switching easier and clearer.

So he's not saying leave them all in there. Swipe away only the apps you don't use frequently.