Topic on Glossary talk:Game data
We do not really worry about that. As far as I'm concerned, at least using the data path is great. Plus SDL2's SDL_GetPrefPath recommends the data directory, and it's enough of an achievement as it is.
As to FTL, I am not aware of it being a botched job like that - both the config file and the save files are in XDG_DATA_HOME, rather than one always using .local/share as the page implies.
If it uses either path, it's meant to be true. And in fact, it can be both - see Bit.Trip Presents... Runner2: Future Legend of Rhythm Alien, which uses XDG_DATA_HOME, but fails to use XDG_CONFIG_HOME correctly.