Sure, it indexes the text, but that has no bearing on SEO.
Most people using hidden text use it as a keyword stuffing tool, stuffing more relevant keywords into the page in order to get higher page rankings.
A lot of SEO techniques and how Google views them comes down to intent. It's pretty clear using hidden text and other gray and black hat methods will be picked up by Google and penalized because its clear the intent is to try and gain an advantage in the SERPS. This has been true going on ten years or more:
Cassandra — April 2003
Google cracked down on some basic link-quality issues, such as massive linking from co-owned domains. Cassandra also came down hard on hidden text and hidden links.
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.
Sure, it indexes the text, but that has no bearing on SEO.
Most people using hidden text use it as a keyword stuffing tool, stuffing more relevant keywords into the page in order to get higher page rankings.
A lot of SEO techniques and how Google views them comes down to intent. It's pretty clear using hidden text and other gray and black hat methods will be picked up by Google and penalized because its clear the intent is to try and gain an advantage in the SERPS. This has been true going on ten years or more:
source: https://moz.com/google-algorithm-change#2000
Cassandra — April 2003 Google cracked down on some basic link-quality issues, such as massive linking from co-owned domains. Cassandra also came down hard on hidden text and hidden links.