Well if you're deploying a static site they are the same, but that's still not the whole picture. They have support for lambda style "serverless" functions and Fauna DB[1], and can bundle functions with apps automatically for some tools like Next.js to do server side rendering for dynamic routes[2]. So while they don't support quite the same level of custom stacks, backends and DBs, they do provide tools that enable full stack applications.
Heroku is similar, but Netlify is at least equally simple I found. Maybe someone else can shed more light on differences.