My exploration of the world of cryptocurrencies and blockchain technologies is relatively recent, but the problem addressed by this QRL (which I was unaware of) had already crossed my mind several times.
From what I understand of the cryptographic hashfunctions underlying the mechanisms for establishing consensus within the blockchain and preventing the corruption of the system, these rely on the fack that brute force methods (whether raw or optimized) to attack the blockchain would be temporally and energetically unfeasible.
Quantum computation works, in principle, as if many parallel calculations were performed simultaneously, making quantum algorithms able to tackle heavy problems which are computationally hard for classical computers. It might be possible that the cryptographic standards used in most blockchains would easily be cracked by quantum brute force algorithms, which can be a serious security issue when quantum computation technology gets to a more mature point in its development. Actually, most of the cryptographic measures of security which are currently in place everywhere will have to be updated before quantum computers kick in full throttle. There is even a whole field of study in Quantum Cryptography.
It's interesting that QRL is trying to beat the competition in this point. But since quantum computations poses a global threat to all classical cryptography, and since commercially available quantum computers aren't expected any time soon, I would guess that there will be plenty of time for all systems and institutions to implement updated quantum cryptographic measures.
Well said, I am definitely not an expert in the field of Quantum cryptography. My general understanding leads me to believe the threat is real however not something that is imminent.