TeXmacs is more mature than SILE, or any other recent attempt to alternative typesetting engine. It exists since the '90 while SILE seems to be around since 2015. And TeXmacs is a complex and complete piece of software which includes a document format, a typesetting engine, conversion engines from and to various formats, user interface, a graphics editor (https://twitter.com/gnu_texmacs/status/1607316217206898688), a plugin system to interface with other softwares. All this is written in a mix of C++ and Scheme, extensible in Scheme and the typesetting engine comes with a proper macro language with first class macros (i.e. macros can be arguments to other macros). Moreover TeXmacs has other subsystem like a parser for mathematical formulas which can assign semantic meaning to user input (e.g. the "-" in "-1" is different from the minus in "1 - 2") and a subsystem which generates mathematical glyphs to complete usual fonts for their use in mathematical typesetting (https://twitter.com/gnu_texmacs/status/1606953318118756354).