First of all, welcome to the forum!

The flights you refer to are called 'open jaw', which means returning to or from a different airport than where one originated from. Double open jaw would be doing the same trick with different airports on both sides.

I'm not so sure how they manage to find such low fares. I do know that I run into something called an 'error fare' every now and then. I also keep an eye on the mileage run section for good deal. Most people just seem to run into it every now and then and post it here to share a good deal with us.

I do know that there are many possibilities for tweaking itineraries based on routing rules and fare classes. I don't think they actually would brute force themselves through several booking engines. That'd create a suspicious amount of data traffic and might slow websites down or, worst case, shut them down as if it was a DOS-attack. They might actually search some search engines, or have a special link in Amadeus.

At least I do know that it's not a human behind the buttons. I just ran into a deal with AB that goes from 'Berlin' to 'unknown'.

I just hope the site isn't a scam...

EDIT

I just took the time to test a few of their miracle fares and it seems to be a combination of rock bottom deals and error fares. I checked for example a fare from WAW to SSH in september. They sold it for PLN 1069, LX sold it for PLN 1081.