If I understood correctly, both releases are based on the same Ian Coleman tool. The difference between them is that MrFlay's version performs a checksum calculation, yours doesn't. Anything else?
That is indeed correct... mine was basically identical to Ian Coleman's with the checksum stuff effectively commented out (or forced to return "true")... and the BIP39 passphrase changed from "passphrase" to "electrum".
The FarCanary version was slightly more robust and actually implemented the Electrum checksum code.
Has anyone used the Electrum Seed Tester by MrFlay and checked its code that it doesn't do something it's not supposed to?
I had originally given it a "relatively thorough" check and couldn't see anything out of the ordinary... but it certainly wasn't a line by line check, so I wouldn't stake my life savings on it.