Earlier quoted context omitted.
So if I'm reading that page correctly, the basic gist of the claim is: If we only do a half assed job of sanitising user input by attempting to blacklist whatever javascript we can think of, we'll still be open to XSS attacks from people smarter than us who put css into our user supplied data, so the answer is to prohibit inline CSS - not to properly sanitise user supplied data. I think there are better pieces of sec…
Why not do both and be safe if your sensitization has a bug?
The vulnerability means they can inject arbitrary markup including or that load offsite sources.
You can use CSP to whitelist allowed offsite domains. But if you're not careful, "you never know" and "you might as well" are more likely to waste your time chasing low value things.
For instance, inline CSS is valuable as an intermittent developer convenience, and disabling it takes that away while protecting your from an unlikely event.
Also, you generally should be escaping-by-default and not sanitizing. A templating system should escape by default and make it obvious when you opt out.