IIRC TeXmacs supports only quite limited subset of what LaTeX and TeX can do. Just like LyX, it could create new documents but will often fail opening ones that were created outside of it.
"corpus-scale conversion work aimed at 90% error-free HTML (currently 75%)"
although there may be issues that I do not understand or did not see (I looked at the paper very quickly) that make it more difficult for the authors than for the simplest possible translation.