No, when Scratch first came out it was a stand-alone Squeak Smalltalk image that you ran locally. There was a Java-based online viewer, but no editor.

Then it moved to a Flash version. If I remember right, this was at least somewhat motivated by vendors dropping Java support in the browser. Of course, at almost the same time the Flash version came out, vendors started dropping support for that.

The current one is written in JavaScript, I believe. Edit: it's still running Squeak, but using a JavaScript Squeak interpreter. See the top-level site for the given link.