My spontaneous feeling is that it's very upsetting that it has TeX in the name but does not use Tex or Latex.
You can edit each part of the rendered equation in realtime, without having to write source code as in LaTeX, similar to MSWord, but with the typesetting quality of LaTeX and with the movement keys of Emacs.
This is how texmacs describe it themselves: “Converters exist for TeX/LaTeX and Html/Mathml. Notice that TeXmacs is not based on TeX/LaTeX.”
To me that sounds like a tool that is not for working with Latex, but it can convert in a pinch. It's a different category, it's own thing. Fine, but not Tex/Latex.
The “Limitations of the TeXmacs to LaTeX converter” from their webpage sound pretty severe, aligns with this view.
It does use LaTeX: it contains a usable LaTeX importer/exporter. Yes, the importer and exporter aren’t perfect, but they are used in a nontrivial capacity nevertheless. LaTeX is such an integral part of the research workflow for so many academics that leaving it behind entirely is not an option. So instead many people use TeXmacs to generate LaTeX for submission e.g. to a journal. To me, this means that it is a LaTeX tool. Maybe this is not exactly the intended use case, but here we are.