It isn't an edge case in the SAAS world, it's just that the majority of SAAS tools either don't implement mobile, or don't do it well. Every single SAAS company I have worked for in the last decade has had complex web apps that required very involved HTML and CSS work (and a dash of JS) to get them to work well on mobile. Additionally very few of the front-end or full-stack devs on those teams actually knew HTML/CSS well, they were all React/JS-first.
Sure the majority of websites on the web aren't SAAS tools, but the nuances of mobile vs desktop layout described in this article don't apply so much to such simple content pages.
Most SAAS tools aren't really used on mobile devices. Laptop or desktop is the expected use case. You're still dealing with an edge case if you need to have a "complex" SAAS tool that is mobile friendly.
Sure the majority of websites on the web aren't SAAS tools, but the nuances of mobile vs desktop layout described in this article don't apply so much to such simple content pages.