It's a "this transaction is valid even though the signatures, amounts, potentially everything is wrong about it" vuln.

Every node that uses this library to validate would lose synchronization with every other node (if we take them at their word that it's not a monoculture), the bigger half would be considered "correct" according to how blockchains work, if it's the non-exploitable half - just lots of wasted resources and longer settlement times, if it's the exploitable half - illegal transactions would need to be reverted by agreement of the community, which is some sort of reset.