The last line is related, but is based on some faulty logic. The penalty isn't going to be evident if you're doing a search like:
in:mysite.example.com "explicit text search"
All you're seeing is that your site was indexed. The author posits that since the page is visible in the index, there was no penalty. That's not true at all, of course. If there were 1,000,000 results for a more generalized search like "explicit text search," the author's pages could well be very, very low and that could very well be due to gleaned greyhat techniques.
As another commenter put it, Indexing != SEO. Google has for years been quite dedicated to sniffing out webspam and penalizing those who use black hat techniques.
You can't hack SEO. It may lead to some temporary gains - but inevitably the house of cards will crumble, and you'll find yourself having burned the domain you built with penalties.
TL;DR - If you hide something on a page, you may in fact be indexed. But in terms of rankings, your content will likely wind up on page 9,743 of the SERPs.
Thanks to the OP for the original article, though - I'm giving a talk on 'SEO for Developers' next month, and this is a great example of false conclusions and how NOT to approach SEO.
"So here You have it. Google can index (and indexes) hidden text and dynamically inserted text."
Then concludes the entire thing with something completely unrelated:
"If content is relavant [sic] to Your website, You won't get penalty from google."