That’s the developer of that app, I’m sure they know what they’re saying.
To be clear, intention doesn’t equal to observed execution.
It is *intended* only for original vanilla games, and *not* for ROM hacks. If it works for the ROM hacks you’ve played, then sure, it’s a happy coincidence, that is due to the fact that the ROM hack hasn’t changed enough of the data surrounding the save to deviate from the original games. If a ROM hack save deviates enough from the base games, PKHeX won’t identify it and work with it.
And also, some fan games aren’t even ROM hacks, like they’re straight up creations from RPG maker or something, so the save format isn’t even close to anything PKHeX would identify and work with…