React can be run from a single file from a CDN, and used without a build stage. Only catch is you don't get JSX. While by today's standard some people might consider that unacceptable, it was a fairly common way to use it when it was new (because relatively fewer teams used builds for JS back then, and JSX was far from being widely accepted).
My first year of React development was without JSX even though we DID have a build step (I was categorically against it at the time. Things changed, haha).
I mean, using Babel in the browser is essentially a build step. Your code isn't executed as is and has to be preprocessed. It's huge and slow. Not particularly viable. If your goal is just to play around, sure, but the point the Vue folks are making is that they can do the same in a way that is production friendly (maybe not scalable, but still production viable). This isn't.
My first year of React development was without JSX even though we DID have a build step (I was categorically against it at the time. Things changed, haha).