Block may mean many things.

If block is "blocked people can't see your content" then sure it's just not possible on a public internet.

If block is "blocked people can't comment on your content" (which was mentioned in your quote) then it's not that simple. Website do have various forms of blocking (for comments, posting etc). Of course that doesn't "block" someone from commenting elsewhere and linking to your post, but that's a different thing.

It's almost like people feel "ownership" of their posts, and when someone replies, they almost feel like people have posted "on their space". So even muting doesn't feel like enough, they want no one to see bad stuff "on their space" (in replies). They feel as if they were talking in a some cosy space and then someone intruded with their bad stuff. You'd be upset IRL if someone interrupted your conversation with some BS, even if your conversation was happening "on a town square". IRL you could tell the intruder to get off or you could go away and stop the conversation, but it's not possible online - they can keep yelling at you forever and can invite friends, etc.

Maybe muting feels like not enough because if everyone can see how you're not responding (if you muted them) then everyone might think "if he has nothing to say then maybe their BS has merit". If you muted and not responding then for others it feels like you just have nothing to say, and that's not true. Maybe if there was a marker visible like "this person is muted by the person he's replying to so take that into account" it would help everyone understand the situation better and the urge for "blocking" would be lower. 

Anyway, it's definitely a mismatch btw what's possible online and offline. You wouldn't let some idiot follow you in all public spaces IRL and yell at you, you'd punch them or call police. Or you would at least try to let everyone around know that you're actively ignoring the idiot. But what can you do online?