Because this post is
specifically a found private key for a certificate in the Web PKI it was not necessary to post that key in order to achieve all the
positive consequences of public disclosure. The key only enables
negative consequences.
I could give some leeway to a grey hat who finds the data but doesn't understand what it is and posts it. "Hey, what's this blob of data?". But this poster clearly understands it's a private key for a certificate in the Web PKI.
You can disclose the fact that you know a private key that isn't yours without revealing the key by various methods, but one that's very easy for non-experts is to create a bogus CSR. Just tell OpenSSL (or a tool that's actually good) that you have this private key and you want to request a certificate for it. Give bogus details, for example in the Common Name of the proposed certificate you can explain this is a Netgear key you found.
You can now publish the CSR, it is inherently proof that you've got Netgear's private key but it does not contain that key and so black hats will need to do their own work, for which you are certainly not responsible, to get that key from a Netgear box if they want to.
It would also make sense to tell the issuing CA, you should send them that CSR, although it's not terribly harmful to send them the actual private key and I guess if you're worried the CA's representatives don't "get it" this is a very blunt way to make your point. If the issuing CA doens't respond, tell m.d.s.policy both that you found this key and that the CA did not respond, and if necessary that can be escalated until the CA is distrusted (by Mozilla, and in my experience eventually everybody for reasons we'll not think too hard about here).
In the case of Let's Encrypt the process is, like everything else, fully automated. Call the API (by running your client software of choice) and prove you know the private key for a certificate they issued and want it revoked, its status changes to revoked and the next batch of OCSP signatures will show revoked for that certificate.