It looks like the mozjs version needed by pyrogenesis isn't normally provided for Ubuntu 20.04 (Focal Fossa). It needs version 78, whereas I have version 68 installed. That package web page does show that 0 A.D. ships the correct version for Focal Fossa, but I guess that's only if the game is installed through apt, not flatpak. I'm not sure how to get the correct version on my system though, since it depends on an updated version of the libc6 and icu packages, which throw errors when I try to update.
Maybe the solution is to install the debian package for 0ad, which is still on Alpha 23, for the scenario editor, but use the flatpak version when I actually play the game ¯\_(ツ)_/¯