I hear this a lot, but it seems to me true only in a limited context. "Walls of text" are disastrous in marketing and some technical content, but these should not be the standard to which all writers aspire, even if they publish exclusively online.
For example, the London Review of Books[0] is famed for its long paragraphs, but they suit the topics and discursive, nuanced argument. Chopping them into smaller chunks would not make the arguments easier to follow for the educated readers who subscribe to the LRB.
[0]: https://www.lrb.co.uk/the-paper/v42/n23/james-butler/failed-...
Note that the paragraphs are not indented, space-separated, and the text is left-justified.
The use of a serif font is somewhat unusual for a digital medium, but it’s a very crisp, “light” font, presented with maximum contrast, on a pure white background.
That is a concession to digital media.
Also, the writing is excellent, which helps a lot.
Basically, I think we've all encountered sites that have huge blocks of text that could be broken into discrete sections (AKA "paragraphs").
Paragraphing is a bit of an "artform." All of the rules are heuristics, not "hard and fast." I feel that it helps my prose to be more readable, if I break it up.
One reason is that, even though I am fairly prolific, I am not a "top notch" writer, so I need all the help I can get.
As a result, they can stick to providing long-form writing instead of chasing whatever the new "maximize engagement/conversions" trick is.
I still believe in short paragraphs; though not necessarily single-sentence ones. Paragraphs are meant to collect ideas, and it’s often a good practice to have a fairly “granular” approach, with “atomic,” self-contained “modules.”
The idea is to allow reading to proceed in a “piecemeal” fashion. This is due to the way people consume prose, these days, with sidebars and interruptions. It also lends itself well to reference reading.
In any case, a “wall of text” approach is disastrous in digital media. It works well, for justified paperbacks, but not so well on a digital device.