It works but there are some issues to be worked out when customizing many settings in the Site Editor, and after you switch to a different json file. Let me explain:

My initial json file was not recognized initially (long day), I ended up copying the initial theme.json file provided by the theme (Hansen is my fav. Block theme - for many reasons...), and made some setting changes to the code to the copy, among other things changing the version numbers to 2, instead of 1.

At this point, Gutenberg finally recognized that there is an additional json file which I could swap out easily. So I did, and customized further (mainly colors of elements) with the Site Editor.

Now this is the tricky part. When I switched back to the original theme.json file, all my previous editing done within the Site Editor were gone, and all the default values stored in the actual file were loaded. This keped repeating every time I swapped the json files. This tells me that in the database, the modifications made to each style were NOT stored separately for each file. If not corrected, it could be wasting time, especially if you are testing different styles and customizing many things within the Site Editor, and going back and forth with different json files.

Another issue that I came across, as an example, changing the color of the Group block which is in the header area, will remain the same even if you make the color change at block level in the Site Editor. If you swap the styles in this example, to automatically have the "correct" header/footer color you have to do it either in the json files, or for each global styles Block customization in the Site Editor. That also causes an issue though... If I wanted to have different colors for the header and footer, this is not possible. Also let's say in your json file, or later on in the Global style editor within the Site Editor you make the default background color of the Group blocks black, then each time you use the Group block in the content area (post/page/CPT), the background will always start as black, which in many cases you have to manually fix it on block level. A good remedy would be able if we could also set things according to classes not just blocks - is that even possible now?

Nevertheless, to almost any end user, the whole Site Editing experience will be a nightmare - they need to understand many things, file hierarchy, the loop, overwriting correctly CSS settings (Site editor level, block level), etc...

Also, after using the Site Editor, I can export my templates/template parts, and merge them in the initial theme, and package them as a new theme for distribution, why can't I do to the changes I make to the Global Styles (theme.json)?

Overall though, very promising start, I am impressed thus far... This is one more important feature when done correctly will eat something that non FSE premium themes were doing for years...