I mean, just look at their example. Level 17+ for the math? Holy shiiiiiit. At levels 15+, you're basically living legends that are about to start conquering kingdoms singlehandedly and fighting gods. That's not "a talented musician," that's the dude who leads month long rave parties that brings in all the nobility's children and starts a religion that is then used with said children to start coups in the continental region.

Ignoring the actual rules and mechanics is basically step one in almost every "isn't this goofy" D&D anecdote.

Not only is it not "decent damage" (even the buff it got in 5.5 just brings it from "the worst" to "poor"), it's also not a subtle thing you can just drop on someone unsuspectingly.

Spellcasting for an attack is an obvious aggressive action, which means an initiative roll comes first to see if you even manage to get it off before they clock you. It's also not like everyone around just shrugs and lets you go about your business because all you did was hurl an insult. You attacked someone with an offensive spell, the response is exactly the same as if you threw a firebolt at them

The flavor of insulting someone to death is fun, I'll grant that, but there's nothing special about Vicious Mockery mechanically that makes it immune to initiative order or people noticing what you're doing.

It got buffed to a d6 in 5.5e

But then again, with this math they're talking about 5e, so your point stands