I suspect the audience is people with a React site already but in that case I'd expect they already have a landing page.

These corporate sites are really the ones that should just be old fashioned HTML + CSS if possible.

eh, not necessarily, you could claim using a popular framework has it's benefits. It's easier to transition it to a full app whenever the time comes. The author could've chosen something less hip?, sure, but again React is widely used and it has proven to be useful in many app scenarios.

Because you want your landing page to load instantly and React or any other JS-generated site just can't compete in loading speed with a static HTML page.

Because you need a bit of CSS knowledge and to know how intersection observers have to be configured and then everything works with few lines of JS and CSS ... it's lightweight and SEO friendly and not bloated as if you use REACT.

Uhm, probably not the best idea to mix your saas application, with your business website. I would definitely not recommend that.

And i will also echo the other comments, saying that this is definitely overkill. If you're doing a single landing page, jsut go with static html/css. If you need some content management, then either go with a fully fledged CMS or do the hipster thing and compile into static site.

IMO landing pages and SaaS applications should be different projekts/repos