Their protocol only uses cryptographic hash functions, which means that it's post-quantum secure. One reason why this is significant is because existing post-quantum public key algorithms such as SPHINCS+ use much larger keys than classic public key algorithms such as RSA or ECDH.
*Edit: As other have pointed out, for SPHINCS+ it's the signature size and not the key size that's significantly larger.
*Edit: As other have pointed out, for SPHINCS+ it's the signature size and not the key size that's significantly larger.