I think the point appended was trying to make is that there are role-players who deliberately cross that line. Which is true -- just as the point you made is definitely, without doubt true. And between point A (appended's 'Frank's player thinks I'm a [whatever insult you'd like to apply here], so Frank-the-character is being used as a way to bully me') and point B (your 'Frank-the-character doesn't get along with my character, so they will butt heads IC with no OOC insult meant') is the attempt to determine which is what.
I've encountered situations where a player was not looked upon fondly by OOC cliques inside a game, and IC harassment was used as part of the aforementioned cyber-bullying. It happens, we all know it happens. And because the stereotype of RPers as passive-aggressive socially awkward penguins is often at least a little bit true, it is easy for that sort of shitty behavior to spiral out of control if someone doesn't step in. What it comes down to is a serious examination of the situation, and then handling it like adults. For some people that means hiking up their big girl panties and dealing with the much-dreaded confrontation -- 'I have noticed X. This makes me feel Y. Let's hash this out and put it to bed.' For others it means putting the catty schoolyard behavior away. And sometimes it's just reminding people that OOC ≠ IC. Everything is situational, subjective, but what it invariably comes down to is acting like empathetic human beings who share a love for the same hobby.