Substack uses dark patterns in a pretty random way (different dark patterns modals on different blogs, often different dark pattern modals on the same blog at different times) to confuse people about the nature of the site. They'd rather you be confused or intimidated to sign up for something and try to make it very clear that you can read articles online and that your refusal to subscribe to a newsletter could be permanent and not just a "not yet" situation.

Unfortunately this is endemic to the industry. Try using Windows sometime. Oughta be a law that any "not yet" button for a thoroughly pizzled [1] product or service ought to have a "never" button next to it and the font size of never has to be 3x larger than the "not yet" and the contrast of "not yet" has to be less than the square root of the contrast on "never". That would be about fair.

[1] https://en.wikipedia.org/wiki/Autofac