Quantum bit level security of Supersingular Isogeny based Diffie Hellman scheme
Block cipher is a concept from symmetric cryptography. Here we're talking about public key cryptography. SIKE is a public key encryption (PKE), and a key encapsulation mechanism (KEM).

Of course Grover's algorithm applies to any public key cryptosystem, but there is not a single system where we don't know a better algorithm than Grover's. SIKE is no exception.

The quantum security of SIKE is (roughly) obtained by dividing the bit-size of the prime p by 6. For example SIKE503 has about 80 qubits of security, but NIST security categories come by large increments (64, 96, 128 qubits), the closest one being 64 qubits.

There is currently debate on whether the divide by 6 rule is too pessimistic.