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-...