Somewhat an outsider (but I have used Netlify and eleventy). It's basically a tale as old as time: trying to build a sustainable business around open source technology and perhaps fleeting trends in software development.

Vercel and Netlify offer essentially the same thing: easy hosting of mostly static websites + a few bells and whistles. They each support development of frameworks (eleventy, Next.js, Gatsby, and perhaps others) that make their hosting business more compelling to use. Netlify branded it's approach as "Jamstack" which I believe stood for "Javascript and some other Marketing Stuff". Now that the winds of change are blowing, Netlify want to reposition themselves as something a bit different and down play the old stuff and also not make it look like their rivals are doing to well.