I'll add that "sell yourself" has (Unfortunately) negative connotations in a Western context. (Possibly in other contexts as well, but I don't know.)
We teach children modesty. We correct people who brag. We emphasize the "everyone is equal " approach. Which are are correct things to do as children. Children's work is encouraged (regardless of quality) and is discarded.
But this mindset can work against adults. Primarily because the work is expected to be fruitful, not discarded. And for work to be fruitful it must not just be fine, it must be seen.
At my company we have a saying "if it's not documented, it's not done". (I write software libraries.) If I don't write docs, build examples, publicize new features, then no one will use them. That's a waste of my output, and a waste of the money the company spent for me to make it. This is not a "modesty" thing, it's a "that's my job" thing.
Now, can you write code on your own time, stick the result on the fridge, and admire it yourself? Of course yes. You don't have to publicize it. It's perfectly OK to have a hobby. Frankly it doesn't matter if it's good or bad. It'll serve its purpose and be forgotten.
"Selling" in this context may be the wrong word. "Documenting" or "Publishing" would have served a similar purpose without the stigma. But the author wants to stress test there's a "persuasion " aspect here as well.
My library might be better, faster, more secure, and so on, but there are other libraries our there competing for users. If I won't "persuade" others to try it, who will? If I'm not prepared to stand up and advocate for it, what that does that say about my perspective of the work? If I don't believe in it, why should they?
If you don't think your work is worth talking about, so be it. Just don't expect anyone else to talk about it either.