Simple, there were just few cards in testing phase. When few thousands cards go into, no way you can conceal them from being spotted by Mastercard - you can conceal small mouse,but not the elephant. 
I think there is another possibility.
They could easily have just taken an off the shelf prepay Mastercard and used that card number to show transactions using API in their web wallet. The "test" card did not have to be supplied by the third party and was only called a "test" card by the scammers while they ran their own tests.
In all probability they used a prepay card and added balance to it, simple using an API. The process just takes longer so probably that is why in those videos of transactions taking place in shops and ATMs they kept refreshing the page on the phone to update balance but it usually took 5 minutes to update. It is probably because it is following a longer route. Instead of going from:
customer >>> card
card >>> customer
they are probably going from
customer >>> third party real card owner >>> card
card >>> third party real card owner >>> customer
Also IF the Minexsystem/Minexpay/Minexcoin scammers really are going to send out cards it seems that these cards are purchased in bulk, their wrapping will have to be opened, their card numbers logged by Minexsystem/Minexpay/Minexcoin scammers and then the cards will be sent to Minexpay customers.
There is also a possibility that the card issuer of those cards purchased in bulk does not even know what scam the Minexsystem/Minexpay/Minexcoin scammers are about to pull so could be completely innocent. maybe they have no dealing at all with Borys and co because they were purchased from local shops and not directly from the card issuer. I would not put it past Borys and co to do that.
As soon as Mastercard are informed about the scam they will cancel all the cards AND they will make sure whether it is Minexsystem/Minexpay/Minexcoin or Chain2Pay - any request for creating and selling cards related to these scammers will automatically be dismissed, Borys and co will be blacklisted by VISA and Mastercard to ever apply for a card program.
This will all end in disaster for innocent customers who will be left with worthless and useless cards on their hands but hopefully it will end up with another court case against Borys and co