Yeah, it looks like four now they're only doing joint official extensions. Basically you gotta work with someone at Microsoft to have your extension added to the store. Otherwise you can create a regular chrome extension and side load it, and it works like 75% of the time.
They simply state "Microsoft Edge supports a new HTML, JavaScript and CSS based extension model. This new model is Chrome-compatible which means that existing Chrome extension developers will be able to migrate their extensions to Microsoft Edge with minimal changes." but not how to actually add the extension or anything to get going.