Ok so I got the opportunity to talk to @Fervis about the All Gen incompatibility issue, and the reason is that All Gen now overrides most of the pokemon object and battle code using the Mods folder, which means that the aliased methods in the "Ash-Greninja Mod.rb" file also get overridden.
The simplest fix is that if you're using this mod with All Gen, you just rename the "Ash-Greninja Mod.rb" file to "ZZZ Ash-Greninja Mod.rb", which will make it get loaded after the All Gen overrides so that both mods work.
This problem won't exist with 19.6 since 19.6 will have complete gen 9 support in base game, so overriding the entire battle code will not be necessary