In Hybrid key exchange in TLS 1.3, they mention that with a hybrid approach, we concatenate the shared secret from each method (say KEM like Kyber768 + classical ECDH X25519 = X22519Kyer768).
concatenated_shared_secret = shared_secret_1 || shared_secret_2
The idea for the hybrid is that it is quantum safe to protect against store now/decrypt later attacks and that the classical X25519 is known to be good today as a backup in case Kyber is broken.
What I am failing to see is how the key exchange actually works.
How do you do this concatenation to actually be secure against both classical and quantum attacks? Aren't you still doing a single TLS 1.3 key exchange and why wouldn't that be insecure against store now / decrypt later attacks if using X25519 or if you are only using a Kyber768 KEM, wouldn't that be insecure against classical attacks if Kyber is broken?
How does this concatenation provide the simultaneous benefit of both classical and quantum at the same time?