The Ars article seems to suggest that they were able to crack his phones pretty easily
Android uses data at rest encryption, which isn't really useful without a lockscreen PIN/password since data gets decrypted after you unlock your screen the first time after each boot
Although it doesn't appear he was actually using any encryption apps to store material; rather, he used a fake calculator app as password protection. Obviously not the brightest bulb in the drawer.
Agreed, he probably felt safe enough "hiding" the files. Definitely not the sharpest tool in the shed, which is great because fuck this guy
ReVanced if you want the original Youtube app
Otherwise there are dozens of apps which don't show ads, just look arround F-Droid
SmartTubeNext for AndroidTV