What part of the Fixi project is building something like React?

Not react yet, but it is approaching the early 2010 jQuery and Backbone vibes, so give it another 5 years and it will converge to React.

The htmx creator has made similar things for a long time, if anything htmx has diverged from those ideas by taking Intercooler.js and stripping out the bundled jQuery to simplify it and create htmx. See https://news.ycombinator.com/item?id=49496634

Have you looked at the project for which htmx4 is released for? it is basically mvc + toolbelt + network library and all that jazz like it is 2012.