The affected keys are made by Infineon's weird RSA key generator. This produces keys with weird properties, listed in their 2016 paper and more so in the accompanying Technical Report.

The test they released today just checks for the oddest of those properties, if you divide the modulus (the main part of the RSA public key) by some small primes (e.g. 11) and take the remainder then instead of a random answer other then zero (zero would be super bad) you always get specific answers. So the code checks about two dozen small primes in this way.

This probably had nothing directly to do with the attack, but it finds Infineon keys and only those are affected.