Thank you!
YES…I lost some hairs to get the stuff straight forward in the flatpak-case 
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Its not »substituted« its the very same as the global »XSCRIPTCONTEXT« which is initialized inside a Libreoffice-process.
(read the source $(instdir)/program/pythonscript.py )
Nice finding, but I’m not sure where it would be best served from. The flatpak’s /app/libreoffice/readmes/README_en-US (and its translations) is just the same as the vanilla LO version, and presumably obscure to find, and IMO at a wrong level for “programmability” findings like this anyway.
Maybe in the wiki-page, wich also lists other “setup”-information for IDE, and covers XSCRIPTCONTEXT
https://wiki.documentfoundation.org/Macros/Python_Basics#What’s_in_a_Macro