I would guess it is something signed with a private key so that the author of the throwaway comment can later prove they wrote it.

Seems a bit short though.

It's 128 hex characters, 64 bytes. That's exactly the size of an Ed25519 signature (https://nacl.cr.yp.to/sign.html), or just the SHA512 of anything.

An uncompressed ECDSA signature is 64 bytes, which is the length of that string.

It's somewhat common for people in some security circles to post hashes of messages they don't want to post publicly, but might want to refer to disclose privately or at a later date, and have some reasonable proof they didn't make it up later.

This, for example, is a throwaway account. Perhaps there's something in the message that'll let them prove they left that message at a later date, if they desire to.