Answers
The fact that someone figured out how to do this kind of scares me, because that means they might be able to hack any unit they want into their game for IVs, merges and SI. Thankfully I'm not in super high teir arena (17-18 depending on the week) so I don't think I need to worry about running into this.
I'm surprised no one knows about this already.
Feh doesn't save its merges/etc server side, so you can hack the data while playing on android emulator. This has been around since the beginning of the game. The things you can hack are:
Unit ID (Changes a unit to another), IVs, and merges.
Typically this results in a ban (afaik), since feh tracks what you summon and runs it against what you have.
Theoretically, this player could have morphed units recruited into black knights and merged them the normal way, which wouldn't result in an auto-detectable ban.