GDPR is almost exclusively for personal content (as in: name, email address), whereas most of the content on Quora isn't personal. GDPR makes no effort to allow you erase that.
You can certainly take efforts to restrict its movement, as Quora has done here. It's mostly futile, though, especially in this case. Archive.org might not be willing to save it, but I guarantee plenty of comparable services are.
My intent was to put forth what it is describing, which is to allow the user to request the erasure of their information and subsequently that provider should then take reasonable steps to do so.
If that provider, chooses to keep content creators information more central to itself and not allow it to be as widely spread. Then that's more power to the curator and could be something that Quora wants to uphold.
On the same token, just because something has been posted on the internet doesn't mean that the provider of such content is obligated to spread it as far and wide as they want. They are allowed, to take efforts to restrict is movement.