They're wrong, as everyone has something they try to hide (even if that something is completely mundane), but ultimately it's up to each person to decide what they want to keep hidden and what they're okay with having shared more broadly.
I personally, am the most boring and non-social person around (as in Social Media usage). The fact that I possibly, but quite literally have nothing to hide besides my Social Security Number, bank info, national ID info etc.. The usual basically, doesn't mean it's anyone else's business what I do, watch or like.
If person A is being addicted to watching porn on his phone while being in the train, let him. None of my business.