https://trac.wildfiregames.com/wiki/GameDataPaths#Linux
should be added to reflect game save location if you installed by using flatpak. I don' know exactly who or how it should be done but for linux mint the path is as follows:
home/user/.var/app/com.play0ad.zeroad/data/0ad/saves
I suspect other data files are there also. I see a cache, config and a data directory.
No need for others to spend their time looking for this.
If this is the wrong place for this I ask forgiveness and please move it to where it should be.
I believe you can see if it was a flatpak by typing "flat pak list" and looking at the output.