"long tail" typically refers to the tail of a normal distribution - basically it's a sciencey, but common, way of saying "very unlikely event". So, the OP was saying that it's worth spending some time strategizing about the unlikely event that a practical RSA-breaking QC appears in the near future, even though it's still a "long tail" (very unlikely) event.

Honestly, there's not that much to discuss on this though. The only things you can do from this strategizing is to consider even encrypted data as not safe to store, unless you're using quantum resistant encryption such as AES; and to budget time for switching to PQC as it becomes available.

The only thing I know or understand about quantum computing is its ability to "crack" traditional encryption algorithms.

So the commenter is saying that Cybersecurity needs to be planning for a near-world where traditional cryptography, including lots of existing data at rest, is suddenly as insecure as plaintext.