I disagree. You can‘t keep data away from others when it isn‘t safe. Security doesn‘t necessarily imply privacy (as demonstrated by your argument), but making something private is impossible without making it secure. How can you hide something in your house when it doesn’t have a lock and anyone can just walk in? Likewise, how is your phone private if, say anyone can unlock it?

> True, depends on the phone though, some of them cannot be locked again and there's no way to completely fix those phones with a better ROM.

Then you shouldn‘t use those phones for a secure setup. I think we can agree on that. But the author of the article used a phone that is capable of locking the bootloader with alternate ROMs.