No, marketing is not unnecessary. However frequent publishing can have a huge positive effect and for a writer it pays off to invest energy primarily in writing.
Before I start, I want to mention that the blog article, you have linked, is quite old. Things have changed a bit - think of influencer marketing.
Another problem, I have with the article, is that a few examples of successful people shall prove the theory of writing a lot will lead to success. No, a few examples is not nearly enough to justify such a claim. Maybe they were just lucky, or some other factor played a role.
Several failed attempts to market a book won't prove that marketing doesn't work. There is a lot you can do wrong, especially if you as a writer do all work by yourself, meaning the writing, publishing, accounting, marketing. It takes a lot of time to develop the skills for every job. Hence, I argue, the reason why marketing seems futile is simply that people do it wrongly. In the end a writer is primarily a writer and not a marketing expert. Also in the examples discussed in the blog article, it is actually proven that marketing works. One author changed the price of e-books to attract more readers and subsequently sales increased. The price is a critical aspect of marketing.
When you write frequently you practise a lot and will improve - a process that can be boosted by detailed feedback from readers. I have made this experience with publishing short stories on forums, the more I wrote, the better I got. I hear this all the time from all kinds of artists, be it musicians, photographers, movie makers, and of course writers. The more you practise, the better you usually get, and better works have a greater potential to be well received by the audience.
It is plausible that a person who choses to write with the aim to earn an income has also a certain passion about writing. Investing time for something you have passion for pays well. This needs naturally to the observation, that publishing frequently and thereby improving on quality, can increase sales. On the other hand, if you see marketing as a burden, you won't get a good return of investment here.
This means the writer should write and let somebody else do the marketing which is still needed. Today there are so many books or other media content out there - with no marketing it will be a game of luck for people to discover your work. And of course for some authors neglecting the marketing strategy works, that's almost a statistical certainty. And those lucky few sell their grand success story and advise everybody to follow their example, completely forgetting that this strategy will fail for most people.
The problem is finding a good marketing expert to hire. Publishers think often too old-school and aren't a great help either. However, if you find a publisher that suggests to send your book to YouTubers or podcast makers, so they might review it, you could give it a try as they embrace modern media. In the end, it's a bit of a dilemma, when you don't have the skill to do it yourself nor have the money to hire somebody. I don't have a good recipe for success either, and I think there is no universal one. In the end, not everybody can get to the very top.