Difficult to comment without URLs, there's too many potential causes
Unfortunately "[being] a web designer since 2016" is generally not helpful for SEO
But certainly setting up Google Search Console, submitting a sitemap, and 'fetching' your homepage on day 1 is good practice, especially for a brand new website
I can also vouch that what the author is saying is true. I've started publishing my notes, and I found that after three weeks none of them were indexed (and yes I submit a sitemap through google webmaster tools). I have since started using the URL inspector tool and adding them one by one. That does work.
If you're mass-producing low-quality sites you would do that on day 1 with each of them. So it likely doesn't weigh positively as much as we would hope.
Unfortunately "[being] a web designer since 2016" is generally not helpful for SEO
But certainly setting up Google Search Console, submitting a sitemap, and 'fetching' your homepage on day 1 is good practice, especially for a brand new website