The problem is that the people that are a) evil enough and b) technical enough to understand this will sell COVID passes for hundreds of dollars each.
Here in the NL there have been tons of people that sold COVID passes, some working at vaccination places, others working at testing places. Instead of hacking anything, they've just been committing regular fraud. The street value of these passes seems to be round €300 to €500. The government has been blacklisting these passes ever since they were first spotted, leaving many of their "customers" angry now that they can no longer fraud their way through the necessary checks.
The problem is not so much a technical one, as modern crypto is quite unbreakable. The pass in the article is based on RSA-512, which has been proven to be breakable all the way back in 1999. With elliptic curve cryptography the system can still remain unbreakable even with shorter keys that can fit into a small QR code, though space is rarely a problem with these codes anyway.